I know that there're a lot of different card sizes out there, and it's pretty much accepted that the two most common standards are poker size (2.5x3.5in / 63x88mm / B8) and bridge size (2.25x3.5in / 56x88mm). Those sizes are very similar, so I wonder if folk only familiar with one (or both) of the standard sizes might be put off by a game involving cards that are 'too different'. The questions I have (open to all, but mostly targeted to anyone experience in games with different size cards) are:
Have you ever found that certain card sizes felt more natural/appropriate than others?
If a card felt off, was it more likely due to a different ratio (narrow/wide), absolute size (large/small), or material (too thick/thin a cardstock)?
Have you ever been put off a game because an unusual card shape/size just felt awkward?
Have you ever thought that certain card sizes might be more appropriate for certain types of games (trick-taking, CCG, board+card games, etc)?
Do you think publishers are possibly more or less sensitive to card size "standard-ness" than players are?
Have you ever heard anyone else offer an opinion on this or a similar topic? If so, what was it?
I can speak to this from a slightly different perspective. Due to manufacturing limitation, a board game I had produced had to do a non-standard sized card. There is a significant cost difference when you change your card sizes from standard, as well as quite a bit of difficulty in getting any kind of promo work done. So its best avoided if it all possible - though our card size hasn't had any mentions from players yet, and they don't tend to notice unless we point it out. We did, however, keep the standard card ratio the same.
To answer your original question - I can think of at least 1 case where card size put me completely off on a game - it was the original Ticket to Ride game. The cards it came with for the railroads were very very small, and difficult to shuffle well. I assume that I wasn't the only one who felt this was as the first expansion to the game had a full replacement deck for these cards - and subsequent editions did not have the small cards as well.
I tend to notice super thick cards pretty easily (the original CCG release of Illuminati had very thick cards), because of how they shuffle. But aside from that I haven't been bothered by slight changes.
I agree with Yamahako - I rarely see cards that aren't "standard" so I can't really comment on that. However, I do notice what a card is made from first and foremost. For example, the cards included in 'Death Angel' bug the living **** out of me, because they are so wafer-thin. I feel like I might bend one in half just by bridging a bit too exuberantly.
I have played a few games that require me to hold "not cards" in a card-like fan, and that gets me too. Strange shapes (octagons, triangles, peanuts) all irritate me to no end. My sister has a deck of playing cards that are all cut like butterflies. I can't ever figure out how to hold them.
Card size is also dependent on card layout. I've been playing around with the idea of putting flavor-art on the *bottom* of cards. I hate that unless I memorize the art to each and every card, I often have to lay down my hand in order to read certain effects and powers.
Further in that field, cards shouldn't have to be very large. Cardgames are cardgames because they are meant to represent a lot of information in several brief helpings. Nothing slows a game down more than having to read line after line of text on every card of a 7-card hand. If your cards are becoming too full with words, it's time to translate common words to recognizable symbols, and to standardize the most common rules.
There is a lot that you can do with visuals rather than words on a card. Text scares people off, takes up space, and is boring to look at. My game uses color-coded cards to denote what type they are, and effects are also color coded to match the cards to which they apply. Any math is often shown as a combination of a standard symbol, and a number. "+4" (with  being a 2 in a red square) means that you should roll 2 red dice and add 4 to the result. If that 2 were a 3, and in a Blue square, you would roll 3 blue dice and add 4 to the result. The result is that my text is very large and easy to read, and you get a great deal of information from a simple glance. The amount of colors and symbols makes my game look very flashy and bright, and the increased space allows for bigger pieces of flavor-art on each card.
I have elephant hands. I loathe playing Ticket to Ride with the included cards and much prefer the 1910 set because I can hold and see what I have. On the other end, I don't really like the size of Lost Cities cards either, because they're too big. Whether you've got bridge or poker sized cards I don't care. I want to know at a glance and a sweep through my hand what I have to work with.
Along the same lines as above...
1) Straying from the standard sizes can increase your manufacturing costs.
2) Cards that are too small (Settlers' Development Deck) or too large (Lost Cities) can be hard to shuffle and hold.
3) Cards with non-standard *relative* dimensions (like the square cards in the Settlers card game) can be hard to shuffle.
4) The more your game depends on speed, the more you'll want to stay at or near the "normal" dimensions (at least, that's been my experience).
Most objects in our world are built to accomadate our hand size. Remote controls, candy bars, steering wheels, etc. Standard card size is standard for a reason.
The only time I would make nonstandard cards is if the cards were going to be used in the play areas as well. An example is Scrabble.
If you have good reason to deviate from the normal size, I wouldn't worry about it too much. At least for me, it would never be a deal breaker in whether or not to purchase or play your game. I play Settlers of Catan and Lost Cities all the time, and they both have irregular size cards.
That being said, I'd stick with normal poker or bridge playing card size unless you actually have a good reason to deviate. It'll probably be your cheapest option and as others pointed out, they're the standard size for a reason.
I'd be more concerned with making sure the quality of your cards is up to snuff. Cards that show damage after one or two plays make me sad.
I have two games I really like which have different card sizes and I am fine with it.
I have an edition of Illuminati which is puposed to be pocket sized, so it has very small cards. This makes it hard to shuffle, but I don't mind it because it means I can play the game on a much smaller table or surface. It's a trade-off, but acceptable for that edition.
The other game I really like with different card sizes is Ceasar and Cleopatra, which have slightly narrow, but longer cards. I don't think there was any reason for this other than the look and feel of the game overall, but it works. It does add an aesthetic to the game which i appreciate. Again, it makes them harder to shuffle, but I don't mind it because I feel it adds to the game and gets me into the game more.
I've also thought of different card sizes for some games I am looking at. One idea was to use different card sizes for different kinds of cards/ decks. For example, maybe have one "character" deck which has normal sized cards, have an "equipment" deck with smaller cards to play on the characters, and have a larger sized "location" deck so play characters on. I haven't tried it out, yet, but something I was kicking around. I think if it adds to the game then you shouldn't worry too much about changing the size, but if you're just changing sizes to change sizes then why bother?
Does anyone know why CCG cards are a different size (and are wider for their height) than normal playing cards?
If it's because that size is cheaper to print, you'd think that playing cards would also be made in that size.
I appreciate all the responses. It looks like worrying about card size isn't really an issue due to the cost efficiency angle deciding that for you in most cases, and I'm fine with that. My game is nowhere near ready to begin formal production (I'm still drafting rules and making prototypes with 110# card stock and a rotary trimmer), but I know that the complexity of my game could very well hinge on the available card 'real estate', and I wanted to make sure that nothing was too cramped on my prototypes when I sized them to final product size and allowed space for artwork. I currently expect that I'll end up using Poker size cards due to their ubiquity and (slightly) greater size over Bridge cards.