For frontside or Eastern "all-mountain", (i.e. something in a 70-90mm waist) I usually say to go between chin-high and the top of your head. Unless you're racing SG/DH you probably don't need carving skis that are taller than you are. If you want edge-hold and stability at speed, you want torsional stiffness more than something very long.
If you spend a lot of time off-piste, or are looking at powder skis, you need to go longer, probably somewhere in the +10-20cm range vs. your height. Maybe even bigger for really wide skis.
Add length if you weigh more than the average person of your height. Take of a little length if you are a lightweight.
I've also seen things suggesting that length should vary with your height, and stiffness with your weight. e.g. if you're heavier, you should ski more "advanced/expert" skis, but not necessarily bigger ones. Of course, ski stiffness also varies to some extent with size.
If you are talking about flotation in powder, then you would also want to get longer for a heavier skier, so there is more surface area. But you might be better off going wider rather than longer.