Just curious, what size are you going to scale for? What size weapon do you think it might be close to for a normal sized person?
I did something similar with the Tau Pulse carbine. I made the assumption that the carbine was about the size of the M4 and scaled according to that. Worked pretty well, same size as a M4 with stock collapsed, but I cannot remember the ratio I used. I could try to track it down if you are interested.