Chris Farrell of Illuminating Games just wrote a thorough critique of card games setting their body text to be so small that they cannot be read at arm's length, let alone across the table. Here's an excerpt, but the whole thing is worth a read for any would-be card game designers out there. (Myself included.)
Here are some tips for designers who want to stay on Chris' good side.
1: If it can be said in fewer words, say it in fewer words.
Alright, before we get into any matters of layout, the first thing you have to do is look at how well you're using your text. Survey all the cards in your game and set a benchmark for the amount of text acceptable on any card. I recommend no more than two or three lines. Whenever I come across a card with more text than that, I tend to just ignore it. There are probably other, easier-to-understand cards available elsewhere and I'd rather get to play than puzzle over a miniature wikipedia article.
2: If it's said repeatedly, say it in symbols.
Depending on the kind of game you're making, you'll probably have certain phrases that come up frequently. Instead of repeating yourself and wasting valuable text real estate, just make a symbol that stands in for that piece of text.
"Deal two damage to any monster. Deal an extra damage to any Fire-based monsters." can be replaced with [SWORD] [SWORD] (+[SWORD] vs. [FIRE]).
"Draw two cards. Keep one and discard the other." can be replaced with [CARD with an X] [CARD with a CHECKMARK]. There is always a danger in overcoding your visual language, but if executed well, it'll make your game more accessible and playable.
3: Set the right style for the context.
The first thing young designers learn is that 12pt Times New Roman is the Devil. Young designers then overcompensate for years, insisting on 8pt regardless of context. We all have to get over that eventually and learn that there is no one golden rule that fits all contexts. A little card is different from a game board is different from a rulebook.
Here's a simple rule of thumb, though. Up close, set your body text at 10-12pt. Arm's length, 12-16pt. Across the table, 24pt. Always make your leading at least 1.5x the text size. And always, always choose a classic, legible font without any flourishes. I personally recommend Trade Gothic or Frutiger, but both have a distinctly modern feel that might not jive with your game's theme. You might be happier with Garamond or Caslon.
And lastly, if you simply must have flavor text, it's okay for that to be 8pt. It's an easter egg, not critical to actual gameplay.
4: Choose the right background.
Most card games have consistent block backgrounds in which you'll find the rules text, flavor text and any other important words. For the sake of efficient production, that background block is kept to the same size, regardless of its contents. That means cards with little text may have lots of unused space, as Chris notes in his critique.
That being the case, I highly recommend first taking steps 1 and 2 to make sure you're using text as efficiently as possible. With that base established, set your background block to fit your text. You can do this manually card-by-card, but that's a bit of a pain. Try setting a thick stroke around your body text style so your text effectively creates its own background.
Alternately, you can use a 0-distance, high-spread drop shadow. Either way, the background should be high contrast with very little texture or pattern that would interfere with the text.
5: Choose the right art.
And last but not least, the remaining 80% of card real estate is probably taken up with some gorgeous art, right? Just make sure the art actually communicates what is said in the text. In the chaos of the gaming table, the text "Firehose: Deal 7 damage to fire-based monsters" might get covered up by some other cards or a nacho or something. But if your art actually shows a bunch of fire elementals getting fizzled out by a firehouse, the message is still effectively communicated.
And that's that! Some simple steps you can take to keep Chris from complaining about *your* card game. Personally, I like the design of the cards in King of Tokyo. The titles are all custom designed and there's plenty of room for the awesome, illustrative art. The actual body copy is still a wordy on some cards and could afford to be larger, but for the most part KoT gets card design right.