Illustrator - deck of cards advice

With something that might get rather small I would definitely work in actual size, but you need to make sure you're not making it too small so that it's still reproducable. With vector artwork scaling up is not your problem, but scaling down is. When details get too small or lines get too thin, they will vanish in print. Discuss with the printer what is the smallest size and the thinnest line they can produce.

Danica,

As I (mis)understand it, in addition to what Bobby said, (the parts used of) the images of the people must have a sufficient resolution at the printed size, and you ought to start out in CMYK Color Mode (unless the cards are to be printed on printers based on (conversion from) RGB colours).

If you're concerned about visible pixels, I have to ask is the artwork vector-based or pixel-based? If it is pixel-based it needs to be designed according to the output resolution the cards will be produced. If all of the artwork is vector-based there won't be any concerns about resolution. The art will simply print at the maximum resolution of the output device. The main concern would just be making sure the card artwork is set at the right size or at least the right aspect ratio.

If you are having a print shop or other kind of service provider print and make the cards for you I would recommend talking to them. Generally speaking, I think it is best to work at actual size if possible. That shouldn't be a problem with playing cards. Unless you're doing something unusual with the designs bleed shouldn't be a factor.