The file in the above link has now been moved to an unknown location.
To re-size a puzzle slot, in my progeCAD program, this is how I go about it.
-choose MODIFY
-highlight one side of the slot
-click OFFSET
-input the DISTANCE TO OFFSET example .125" (if original slot was .250")
-click where you want SIDE TO OFFSET
The above will give you a center line for the new slot.
Then DRAW NEW SLOT around the center line, to desired size.
Delete the old slot & the center line you drew, which leaves a new slot the size you drew.
It is easy after you do it a few times.