You could consider using alternate blocks to take up some space.
A 12 inch finished block will only cover 144 inches of surface no matter which way it is placed.
The diagonal of any square is approximately 1.4142* times the length of a side.
(The square root of 2 multiplied by the length of the side)
Examples:
12 inch (finished) side - 12 x 1.4142 =16.9704 inches - round the answer up to 19 inches
8 inch (finished) side - 8 x 1.4142 = 11.3136 inches - round the answer to 11-3/8 inches
6 inch (finished) side - 6 x 1.4142 = 8.4852 inches - round the answer to 8-1/2 inches
I think it's worth multiplying by four decimals and then rounding the number to a "doable" measurement.