I don't think theres such a thing as a guitar smaller than a deck of cards.
But if you're looking for a smaller instrument similar to the guitar you could go for a Mandolin or Ukelele, I'm sure you can find one at your nearest pawn shop.
Or you could get a lap harp.
It's a folk instrument, a modernized version of a Psaltery, which is an ancient Zither that was popular in Medieval Europe . I got one for Christmas, it has a very nice sound and it's extremely easy to play, and cheap too.