pdog, it depends on what you normally do, as far as tablet size is concerned.
I have a little Graphite and I'm as happy as a pig in...well, you know
But here's something from Wacom that MAY help:
http://www.wacom.com/tabletwizard/index.cfm
Take that as a starting point, only; remember they're trying to sell you stuff.
As a web designer, the little Graphite is perfect for me, and for the unusual times I have to work on huge files, it still does the job. If I worked on the large files most of the time, I'd invest in a larger tablet. Better than trying to draw and paint with a mouse any day (sort of like trying to draw with a bar of soap!)
Bottom line: no one can tell you what's "perfect" for you. You alone know how you work, what you work on. A large tablet is great if you work on large files (and by this, I mean image dimensions, not file size).