Personally I would stick to poker size if at all possible. When I started I used the bridge size as I thought my hands were too small. They weren't it is just a matter of practice.
The benefits of using poker sized bikes are that you will be able to do deck swaps with gimmicked decks and also will be more relaxed if somehow using a borrowed deck.
Also the following site sells cheap bike cards
http://www.magicbox.uk.com/