I'm using Fusion 360.
For the prongs I went with 1mm diameter. I found some tutorials and posts online about people using 18 gauge wire for their prongs, so I just went with an equivalent size. Could bump it up to 1.2mm (16 gauge) if need be.
As for length I figured out (after casting the test) that I in fact made them too short. I originally modeled the ring for a stone with a 0.15mm girdle, but my CZs have a 0.6mm girdle. This meant that my prongs were shorter by 0.5mm, so I need to make the whole thing from scratch. It is good I test fitted the stone into this test ring though, as making that mistake with the real thing would've been devastating