If I understood your question correctly - you are cutting the foundation pieces 6.5 inches square - and the blocks/units will finish at 6 inches square?
What do you want the finished size to be?
Ordinary twin mattress size is 39 x 75
If one adds a 12 inch drop on three sides (I prefer to put pillows on top of quilt instead of covering the pillows with the quilt or spread) then the finished size would be about 63 x 87
subtract 8 inches from the length and width for the four inch borders - 55 x 81
Figure 6 inches as the finished size of each square - 55/6 = 9.18 and 81/6 = 13.5
So if you go with a 54 x 84 inch center it would take 9 x 14 blocks or 126 blocks.
Depending on how you set your blocks - if you make them into diamonds, I would use an even number of blocks both directions.
If the diagonals are all going the same way, I don't think it matters.
I use the "put the pieces on the bed and see how it looks method" after I think I have about enough blocks made.
Last edited by bearisgray; 05-06-2013 at 08:25 AM.