unless you mean they have to be 3 1/2 or 6 1/2 or 9 1/2 or 12 1/2 ...
yes, thats my understanding of the swap. blocks can measure any where between 3.5 and 12.5, based on multiples of 3 - either square or rectangle
for example: 6.5 by 9.5 would be within the guideline