In Phoenix v5, in the edit function there is a 'scaling' option (slide function with final WS being advised as you do it), which lets you scale a larger or smaller existing model to get what you want - I did that to get my 30cc Sbach to where I wanted it to be (default was a 50cc bird), and the weight etc all looks to have come out bang on in one easy step!
Quite handy I must say!