I’m using STI mags. I have used it on a STI frame and a Black Mountain frame. I did not need to any fitting for either frame. I can try and get some pictures. As far as size goes It feels about the same size as the STI plastic grips to me.
I got one of there V3 grips. It just needed a little extra clearance. Just filed the front and back wall a little on the grip where the mag goes and now it works great.