Cheshire, the one thing that can't be predicted with much accuracy is size, stature. I have a fully mature 11 lb F6 who is who is 12" at the shoulder, one fully mature F2 who is 14" at the shoulder, and one 2 yr old F2 who is 16" at the shoulder. The tallest cat is also very long and he weights 20 lbs., the other F2 weighs 14 lbs and has a stockier body than the other two. My F6 is the leanest of all of them, long in body and when all 3 are standing together, the F6 appears larger than he really is because he is a lean mean savannah machine
We have folks here who have very large F3s and folks who don't. All 3 of mine leveled off on growth at about 1 yr old and then had a mini growth spurt at about 1.5 yrs where they put on a bit more height & length and added more muscle.