The math for this is start with the squares 1 1/2 times larger than the finished size.
2 times the diagonal of the size you want divided by 1.414 = exact size to cut large square.
If you wanted a 4" sq double to get the diagonal which would be 8" divide by 1.414 or (1.5 and trim later) = size of starting square to use.
In case I forget the math look at the diagonal on the cutting mat and count the squares.