Actually, it isn't a simple question. How big is your sashing going to be? Do you want more borders around the outside? How far down on the bed do you want it to come? How thick is the matress.
Here is a link about sizes.
http://www.carolina-quilts.com/site/472586/page/99754
Basically, 36 twelve inch blocks sashed with 3" strips would get you to a square queen size of around 90". I like mine longer.
Sorry I wasn't more help.