size will depend upon how big you want the quilt to actually be--and I'm not sure if you mean that the unfinished block(not in the quilt yet) is 12", but you'll lose 1/2" of seam allowance for any block. So if your block once in the quilt is finishing at 11.5" then 7 across would be 80" wide. If you did 8 down then it would be 90.5" in lenght. 80" across would be a little narrow, but the 90" lenght would work if you were not tucking into pillows. So if 7 across and 8 down it would be 56 blocks.