Yes you can.
In the terrains folder is a file "terrdata.zip"
Make a backup of this file outside your CFS3 root and extract the file "compositetexturebudgets.xml".
Open this one in a text editor and edit the first line
Budgets PatchPixelDim="128"
to
Budgets PatchPixelDim="256"
If you have a real good graphics card you can even try
Budgets PatchPixelDim="512"
Save and zip it back into your terrdata.zip.
Be aware that this can lockup your PC.
In that case just return to your backup file
Oh, don't forget to delete the "terrdata.zx"
This one will be re-created when you boot the game the next time.
If you want to go deeper into tweaking your terrain I'd suggest to check
http://www.sim-outhouse.comfor Winding Man's Scenery Revision 4.