For me, it all depends .......
* is it more decorative or more practical?
* size of the person it's for? 5'1" doesn't need the size of a 6'3"; likewise kids or adults!
* is it for one person or more to cuddle under?
* the design/pattern of the quilt?
* sometimes it grows beyond my intent, as one more border, etc. is needed to give it, its best look!!!! :)
Generally I start at 50x70'ish and go from there. Though if it's to be more decorative, it might be 40x60.
So .... it's really whatever you want it to be!