I would have each block be 1/6 of a yard
You could technically have any value you want, but for me 1/6 is easiest because 1/2 and 1/3 will scale up to this like so
1/2 = (1/2)*(3/3) = 3/6
1/3 = (1/3)*(2/2) = 2/6
The diagram below might help if you're still stuck on why I picked 1/6.