It is a bit on the small side for a rifle, and the sight does end up seeming a bit wierdly small, I didn't expect that either. The sight would really be almost useless on this thing as a real-world gun since it ends up being so low and close to the body that you can't really get a sight picture through it no matter what.
I chose to make it that size somewhat arbitrarily by comparing the length to my Tlaloc model (comparison pic here) and scaling accordingly. I went a bit shorter simply because Destiny guns tend to be a bit huge (the Tlaloc feels just a bit big), and also because it is a bullpup rifle, which tend to be shorter in the real world.
If you want it bigger, simply scaling every file up (15% or so) shouldn't cause any issues as long as every piece is scaled by the same amount. If you like, I can probably upload a scaled up version, but its unlikely I would print another one soon for comparison.