The size range in the scissor slice cut symmetric effect is specified in mm, but the number appears to be the number of slices. If I pick 20mm, regardless of the range in my data set, it will do +/- 10 slices from where I draw my scissor region.
I confirm that it is in slices. Could you please enter a bug report at issues.slicer.org? If you could submit a pull request with the fix then it would be even better.
I’d gotten as far as figuring out how much I didn’t know (which was extensive, to say the least), so reading through your fix will be a very useful tutorial for me.