Oh my God I'm so sorry!! I haven't checked DA for ages (obviously)!!! I hope this late(x10) reply still can help you out!
I don't have a full-scale model. But I did it just based on my own preference, like, just how big I'm comfortable with it, how big I want it, etc. Because, pulling this cosplay in a con is a pain in the @$$, so I don't wanna make it too big (to hold, and to wear, if you are doing the shoe-gun also), or too small.
In conclusion, the size is really up to you. I see some cosplayers made the guns much larger than mine, but it's really not my preference.
If you really want a honest opinion, the length of the guns is about the the length of my head (lol); yup, from top to chin.
I hope this would help.