I'm thinking about getting myself a tablet. I'm studying architecture and like to draw a lot, so I was thinking about digitizing everything and buy a tablet. I'm not sure about the size though. The tablet is only gonna be used for sketches and not for Cad applications. So it hasn't got to be an A3. I really don't want to spend that much money on it. Is an A5 tablet gonna be big enough to draw on? A friend of mine uses an A6 for video editing and just for surfing etc. I found it a bit too small. (I sketched a bit on it in Paint.) Wacom Graphire 4 Classic XL A5 http://www.komplett.nl/k/ki.asp?sku=313019
You could always just cut a couple sheets of paper to the size of the tablet and try out the size that way. If you're used to doing small sketches, then it should work. If not, you may find it too cramped. The tablets I've used in the past have all been small, but everything I did on them was small as well -- clipart and small illustrations for print. Nothing grand or even all that detailed.
It's interesting you brought this topic up as I was planning on looking at some. I'll report back on my findings.
I purchased a tablet this past Monday and it's not bad. I'm not sure how accurate you are looking for sketching and I haven't retouched any photos as of yet.