One thing that might be helpful, although if this is to be a surprise gift might be tricky, is to get a sweater this person already has and likes how it fits, and base the dimensions off that sweater. That way you can get a ballpark figure as to length and width etc.
Once you have those measurements, the finished measurements the pattern says the largest size measures up to, and the gauge you knit up, you can do some calculations based on those. It may be something as simple as incresaing a needle size or two (but not too big, you don't want it see-through!) or bulking up on the suggested yarn in the pattern.