hmm.. depends... 13 inches with 1680x1050 would be pretty hard to see sometimes, but I think I'd take resolution over size... I can always break out my glasses....
I had a huge 20" Picture Tube TV that I hooked up to my pc, and its resolution was crap 640 X 480 at 60hz refresh rate (that is an old style crt television's native resolution). And my eyes were killing me with all of the fuzziness, blurriness and screen flicker - you could not read any of the words on the screen except for the biggest. Right now I just happen to be typing on my Fujitsu tablet pc that has a 8.5" screen running at 1024 x 600, but it could scale all the way up to 1600 x 1200 with an external monitor. So I guess my final answer is a 13" screen with a really good resolution.