Not sure if this is an intended design choice or a bug, but if it's intentional it's pretty silly. In the manual, a specific range for SPSCALE and BGSCALE isn't specified. You just get this (taken from BGSCALE, but SPSCALE's entry is similar):
Enlargement scale X, Y 0.5 (50%) - 1.0 (100%) - 2.0(200%) -
