you can do a border any size you want-
it is best to try to make it (go with) the blocks in the quilt- like if you have 12" blocks- some measurement that divides into 12---that just gives it a balanced look- but i have seen quilts that had a center that centered on a bed top= with 18" borders for the overhang- it was a beautiful quilt---
make your borders any size you like the look of- it is your quilt