Another proud owner of a Wacom here, Graphire 3, A6 sized, in my case. Totally worth the €100 I paid for it a couple of years ago.
(I don't use it that much anymore, I have no patience for digital art.. :/ )

A6 is large enough for most users, though if I'm going to buy a new one (some day...) I will buy an A5 to have a little bit more space. But if you're just starting out, a simple tablet, either A6 or A5 will be perfect for your purposes.