I think that depends on the personal preference and need. I personally like the bigger surface because it feels more comfortable and logic (if that makes sense) to me. That is why I got the wide screen format for the one to one mapping. With the smaller tablets you have to "'write' over your previous movements" a lot and it feels weird to me.
However, if you are going to be going mobile with it a lot I would say definitely stay smaller. Mine is the same size as my widescreen laptop which makes it a little unwieldy when I'm moving around a lot, but if I'm staying in one place for a while it isn't bad at all.
Again, I think it comes done to affordability and what you are comfortable with. Just out of curiosity, do you have any computer stores near you that might carry tablets, even non-Wacom ones, just so that you can try one out and see what feels natural to you? That would be the best way to go.