The most common program that is used is Pepakura Designer / Pepakura Viewer.
With the free version of Designer you can adjust the scale of the prop but, you cant save the changes. You would have to get the paid version. With viewer, as far as I know it's just viewing of files.
One of the newer programs out there is Randy Cavanaugh's Armorsmith. It costs about $15 USD. This program has an adjustable avatar that can be used to upload your files and you can adjust the scale on the dummy and see how it would fit.
There is probably more you can do with these programs. I just haven't learned them yet.