Tablet size largely depends on how you work. I have a 6.5" x 11" surface that I hold on my lap, and it's perfect for me. I've worked on an easel-style 3" x 5" and found that uncomfortable. Not only because of the cramped drawing space but also because I don't like holding my arm up at desk level for very long. People who do some painting often prefer a larger tablet since they're accustomed to making large, sweeping arm motions.

I suggest doing some doodles on a normal piece of paper and seeing how much of it you use. If you tend to draw everything crammed into a corner, then a smaller tablet will probably suit you better. If you fill the whole page and find yourself running off the edges, well, you get the point.

Now, by no means is a tablet necessary. There are people around here who do marvelous work with a mouse. It's all a matter of what's easiest for you. I definitely get a lot of benefit from my Wacom, though, and I'd never go back.