One of the most frequently asked questions from my Wacom Intuos video review is that of the tablet size to get: "What tablet size should I get?"
Generally speaking, you'll want the graphic tablet size to be close to the size of your monitor.
A good size to get is the Medium. For Wacom, this is around 6 by 8 inches, or around A5 size. I personally find that to be a comfortable size to work with.
The tablet surface is mapped to the whole screen. That means if you have a small surface, everything you draw will be magnified much larger. 1cm on the tablet surface could translate to 4-5 cm on the screen. There's a mismatch because you expect your short stroke to appear short on the screen but it will come out much longer. It will certainly take time to get used to using a small tablet.
For medium size tablets, the difference of your tablet stroke and screen stroke is not as much, and hence it feels more natural to see your strokes come out the way they are on screen.
For large tablets, the difference is even less. However, I find that it can be quite tiring to use a large tablet because your hand has more distance to cover. When you're drawing in a sketchbook, usually your hands do not have to travel that far. With a large tablet, sometimes your hand has to travel further to reach the menus, palettes which are all over the screen.
Using large tablets with a small screen, e.g. laptop screens, can feel awkward at times and will take a while to get used to it.
Screens below 19-inches: Get the small tablet
Screens above 19-inches: Get the medium tablet
If you want really fine detail control, get the large tablet.
If you have the budget, the medium sized tablet is one that's the all rounder. I'm very comfortable with it even when used with a 27-inch screen.
And check out my Wacom Intuos 2015 review.