My opinion, (not saying it's all perfect)
I believe that size doesn't overly matter for tablets. I'm not sure if you want the smallest one out there (I have a 6 x 8, medium I guess), but it isn't a huge factor. Your tablet generally fixes itself comfortably to your screen, and I think there are different ways to change it how you like.
Each tablet does a similar job, but depending on the type of artwork you're doing, you may want to go larger or smaller. If you're doing Animation, I find size doesn't matter too much. If you're doing serious art with various programs I'd suggest going bigger! It's all really you're choice.