Hmm. You should be able to go (at least) to the maximum texture size your video card allows. Check your TopoFusion.log file - the max is output there.
You know you can edit the size to be other than the presets, right? Try 2000x2000 (or 1500x2000 or something).
In any case, this is the only place where Pro suffers compared to Basic - assuming you don't have a fancy video card. Many cards are limited to 4096 or 2048.
We do still intend to remedy this (and go beyond what Basic can do) by stitching things ourselves.