The box the appears during the command is not unique to the scoot command, but is the generic AutoCAD 'selection' box or something similar.
Click the AutoCAD Icon in the top left of your window, and go into the AutoCAD options. I believe the setting you want to change is under the selection tab, but if not, it is in here somewhere. One of the options will show the selection cursor, and right below will have a slider to adjust its size.
A quick note; If you make the box to big, when you are zoomed out, the box will be big enough to select two wires, and odds are, it will always choose to scoot the one you didnt want to scoot.
Another note; scooting scoots wire numbers and wires separately, so if your box encompasses a wire and a wire number, it will be a toss-up as to which is going to be scooted.