Here is a simple Flash wizard I did quickly.
-Download the Scale.swf
-After starting Mach with your screen
-View/Load Screen (Drop down to select "Flash Screens(*.swf)
And choose the Scale.swf
where ever you have it located.
-After loading the Scale.swf
Load your Gcode file
-You can either scale to size (Scale width, Scale Length, Scale Height) or Scale To Area (scale all)
After getting the size you want
-View/Load back in your screen
I can enhance this method and add a button to your screen and also a "return to screen" so you dont have to reload or load the flash screen....or if you are using a Flash screen you can find it in the tool box...Please be careful and always do a slow test run first http://Http://alphagraphicdesigns.com/help/Quantum/Scale.swf