Sorry if this is the wrong place to post this. I have been reading a lot of the treads on the zone and have seen many where someone asks about scaling the size up or down.
It seems to me that an easy way to do this would be to make temporary changes in Mach 3. For instance, say you have the g-code for a design, but you want to make it twice as big. If our lead screws have 10 tpi then temporarily chang that setting in Mach 3 to 20 tpi. The computer then thinks that it is moving 1" for every 20 turns of the lead screw. But since you still actually have 10 tpi lead scews those 20 turns are actually moving it 2" for every 1" of your design.
I keep thinking there must be some reason this won't work, but so far I can't think of one. Of course, you would have to remember to reset it to the correct setting once you were done.