It's up to you really... Manipulations decks are a must have for a stage performance, however, if you're more interested in doing manipulation in your close up routine, then stay with normal decks.
Anyway, I think it's better to train (at least when you start) with normal decks, then switch to manipulations deck: it will feel so easier.
Concerning the size, it's pretty much the same thing: poker size is harder, but it's more visible, and again, if you train with poker sized then switch for bridge sized, it will be easy
