I have the smallest one there is but since I bought it a couple years ago, I couldn't live without it. Since you're using a pen, it's more like the real thing and it's so much easier to erase parts of pictures as well. You get much more control with the pen than you ever could with a mouse. It also looks really cool in the dark.
See the glowing blue circle at the top of it? This is one of my favorite toys.