I use 800 wide or 700 tall. 800 is a magic number for PBase, if you keep your pic at 800 or smaller in its largest dimension PBase will use your original image for the "Large" image instead of generating its own. That's an important consideration because the resized images generated on the fly by PBase tend to be pretty soft due to the interpolation and JPEG compression they use.
I have high resolution on my monitor so when I first made my Pbase I started at 1024x768 and higher, after I realised that is usually full screen for most (and requires scrolling) I will only do 1024-768 or less. Watch the Pbase compression however it is not that good. Their compression is not the best so I would concur with Jeff Kohn and use around 800.