Brand-wise, go with Wacom.  Size-wise, either 4x5 or 6x8.

I have a 6x8, but I scaled the working area  (yes, it's scalable) down to 4x5, for the reasons RodB pointed out.

A small plus side of the 6x8 or larger Wacom Graphire is that it includes a transparent cover that lets you insert a photo/picture inbetween the cover and the tablet if you want to trace it -- a feature I haven't used so far.

There are also quite a few buyers' reviews on Wacom tablets posted at Amazon.com.