If you know what bed you are using the quilt on, lay the top over the bed and figure out how much you want to add on each side to make it fit that bed, mattress thickness, etc. I like my double bed quilts to come down over the sides where I don't need a bed skirt and long enough to go over pillows. I use 90 x 108 as a typical double bed size to do this. But then, depending on the design I want to quilt in the borders, the amount of fabric I have, etc. that is adjusted. Size it to fit your bed is my suggestion.