SlightlyOffQuilter

if the blocks are in fact set rectangular like you said, 6 inches wide, and 12 inches long, the grid would need to be 9 blocks Across, and 7 blocks down to achieve the 54 x 84 inch size.

edited to add : Since the block size is stated to be 6 x 12 , it may look like they are " square " blocks if each individual section is a 6 inch block, just done two at a time .