I'm glad you asked this question! I have the same concerns as I'm shopping for my 1st quilt as well. I took slugbait's advice and just now measured my el cheapo Black Diamond down quilt. It measures 56" by 60". With longest side running top & bottom, I felt like I had enough draped around me laying on my side. I think for a 20 degree quilt I may choose a little bit wider quilt for more coverage, but 55" seems sufficient.

Worst case scenario (and I wholeheartedly believe this) I can resell it at a small loss- someone else out there wants the gear.