The biggest difference between Poker size and Bridge size is their width. Generally speaking, Poker decks are 2.50 inches wide, and Bridge decks are 2.25 inches wide; thus, Poker players are using cards which are 0.25 inches (1/4") wider than Bridge players. Both Poker and Bridge playing cards are typically 3.50 inches high. Why would Bridge decks be narrower than Poker decks? A Bridge player is required to hold many more cards at one time than a poker player. Thus, the narrower card is appreciated by Bridge players. Poker players are usually holding no more than five cards at one time, and with the explosion of Texas Hold'em, players are only required to hold 2 hole cards at a time.
Which should you choose? You have probably been playing with traditional poker size most of your life, so if you are comfortable with them, you should stay with them. However, if you are required to hold a large number of cards for your game of choice, you should consider switching to bridge size. Generally speaking, men prefer shuffling the larger poker size cards, and women find it easier to shuffle and deal bridge size cards. The choice is yours, both size cards can be used for any game you wish to play.
Any good internet website selling cards should be able to offer you a wide selection of both sizes.