You can increase the maximum size of the Bas Relief by editing the basrelief.tcl file in your plugins directory (usually "C:\Program Files\AC3D\plugins\basrelief.tcl" or something similar).
Edit this file with a standard text editor, and scroll down to almost the bottom of the file. You'll see these lines:
Code:
# MAXIMUM SIZE OF AN IMAGE (width * height)
# This is the maximum number of vertices that will be created from a texture.
# Change at your own risk.
set ddhbasrelief_maxsiz 65536
You can change the number from 65536 (which equates to a 256x256 texture) to whatever you wish. So, if you wanted to have a texture up to 1024x1024, for example, you could change the 65536 to 1048576. Note, though, that an object with this many vertices can bring even higher-end systems to a crawl.
HTH