Sit bones measured ?
Head down to a Specialized BG shop and get your sitbones measured and while you are there, get them to do a foot bed fit. The insoles are also great...
Once you get the base size measurement, you can experiment from there.
Look at it like deciding on a new flash running shoe, you wouldn't spend $200 on a pair of shoes that were 3 sizes too small ? Same with bicycle saddles.
Physical size has nothing to do with it either. You can get a 6'5" rider with 135mm measurements and a 5'5" with 155mm.