I just recently used the Pepakura Viewer Program which is free to print out and assemble a Master Chief Costume for my son for Halloween. The Viewer Program will not let you change the size of the files to be printed. I scaled the files down to my sons height and weight using the scaling function on my printer.
Another thing to be careful about is if you attempt to print out the file and you dont have the exact paper size it was created on it will ask you if you want to rescale for printing. Be careful. I found out the hard way that not all the pages of a file I printed out scaled the same way. The result was a helmet which did not go together properly and I did not know why until I figured it out later.
I guess I will have to buy the Designer Version of Pepakura and see what options it has that does not come on the free viewer program.