i got a wacom bamboo for my bday, and while sometimes i would like something a bit bigger, its never caused me a great deal of grief... and the size actually comes in handy when i bring it around with my laptop...

as for how it works... i find hand drawing is almost impossible with a mouse... and while i'm only a beginner or average in my artistic ability, i find i can draw things fairly well with the tablet (as accurate as they'd need to be for a map i'd say) i think for serious map making... its almost a must... even just for the pressure sensitivity... the "hand drawn" look its pretty impossible with a mouse, but with a tablet, its a piece of cake...

for you... i'd probably go a bit bigger then a bamboo (5x7) since for real drawing it's usefull to not have to move around to get things right... and if price is not that big of a deal, it'll be worth it in the long run