Start by measuring the mattress top.
Decide how much drop you want on the sides and foot of the bed.
Example: Mattress top is 39 x 75
!5 inch drop on sides and foot
Quilt size: (39 + 15 + 15) x (75 + 15)
(69 x 90)
It is a good idea to add a bit to the dimensions to allow for shrinkage.
Even when one shrinks the fabrics and batting before washing, quilting draws up the item. The denser the quilting, the more "shrinkage".
I don't like to sleep under a quilt or comforter that goes to the floor, either.