The two sizes of cards used in Shoot N' Skedaddle are as follows:
Tarot (2.75" x 4.75") = Character Cards
Poker (2.5" x 3.5") = Special Decks and Weapon Cards
These are arbitrary sizes provided by The Gamecrafter (the company which prints/produces the cards when you order them). They seem to match up with a couple of companies who produce card sleeves.
Companies such as Fantasy Flight Games produce clear card sleeves to protect your cards. I have used these on occasion and the only issue I've run into is that the Weapon Deck (71 cards) becomes pretty large! You might want a card box to hold them.