The bigger your screen (or screens if you have one table covering multiple) the bigger of a tablet you'll want. Unless you're amazingly steady, using the smallest sizes on the largest monitors (2560 x 1600) can be difficult. On the other hand, having a large tablet means moving your arm a lot more....it can leave you tired over time.
So far, the medium Wacoms seem to be a good compromise. I'm currently using a PTK-640 and I'm very happy.
Don't necessarily agree with this. My 6x8 is ideal and I run a Dell 30 inch with a second Dell 20 inch rotated to vertical alongside (they're a perfect height match).
For photo retouching I really wouldn't want anything bigger than my 2 current 6x8 Wacom tablets, an Intuos3 & an Intuos4 at an identical workstation alongside. This is completely workable right down to pixel level even when doing extremely accurate selection paths. A benefit of the 6x8 over larger tablets is that it is small enough to sit in front of the keyboard, and still be able to use the KB comfortably. http://www.pwp3.com/nostromo.jpg
However if you are an illustrator or designer the bigger tablets could be a genuine advantage. Serious illustrators will dig deep and go for the extraordinary Wacom Cintiq http://www.wacom.com/en/Products/Cintiq/Cintiq24HD.aspx
Don't be swayed. Go for Intuos over the Bamboo models. You'd choose L glass wouldn't you?