My first one(wacom graphire classic) was small(A6 paper size active drawing area) and that was a really good and convenient size for a tablet that was easy to also take along with a laptop when traveling.
So for your first one I'd be inclined to recommend that, that way even if you end up getting something else 'better' later be it screen integrated stylus tablet or just a bigger one the small one might continue to serve some purpose too. But thats just my own opinion based on my own experience.
edit: it's also worth noting one other potential pro/con of a small tablet, the way you use it(draw on it) is different from a big one, then drawing on big one you would draw/pivot more from your shoulder with broader strokes, while on the small you would pivot more from your wrist. Downside then is that on the small you might strain your wrist more but your shoulder and back less. If you are trying to avoid carpal tunnel problems go for bigger, if you have back issues go for smaller.
