You might be trying to hard to please a prospective "customer."
You really don't need to jump through hoops for this. I think your response to her could be,
"The size of a double bed is.......
This quilt is........
Measure your bed and note where the quilt would lie.
Quilts are made all sizes, often determined by the layout of the pattern rather than by a finished measurement, so only you (the owner) can determine whether a quilt "fits" their bed."
If she still wants it, she'll do the work that you can't very well do from your house!
Jan in VA