What more or less works for me -
Start with the dimensions of the top of the mattress - then decide how much you want hanging down over the sides of the bed - and if you want to cover the pillows with the quilt or put the pillows on top of the quilt -
And as butterflywing said - need to take into consideration also if the people under the quilt are skinny or round.
In our case, my DH has gotten bonily scrawny - and I've gone big-time in the other direction.