Quilts come in a range of sizes, but your fit is going to depend more on your lightweightitude than any empirical factors. For instance, while most sane people would say that a quilt that only comes halfway up your chest is "too short," or "child-sized," there are people who refer to these quilts with endearing terms such as "three quarter length," and "super ultralight." Both length and width will depend on what you are comfortable with, in terms of what you want the quilt covering and how tightly.
If you want my opinion, I'm only comfortable with a quilt that is wide enough to tuck under my body on both sides with no air gaps--one that is long enough to cover over my head and the ends of my feet, again without air gaps. However, I've found that a quilt of those dimensions offers almost no weight advantage over a comparable down mummy bag in the 20-30 degree range, and numerous insulation disadvantages over the mummy bag (i.e. it is impossible to seal it up without turning it into a mummy bag). But I digress...
Quilts are great for warm nights. I recommend making your own with climashield. That way you can adjust the size as you see fit.