What model is it? What exactly does the manual say?

The only reason I can imagine for requiring a minimum wire length is that the extra resistance will keep the surge current below the maximum that the MOSFETs or PC board will handle. That would be a pretty lame design.

This warning is in Castle manuals: "If you choose to direct solder or to shorten the motor wires, you may do so on the Castle Creations CM36 and Monster motors only. DO NOT CUT any part of the wire length from any other motor, regardless of brand or type."