Decide on block size. I like to work with even numbers so let's say 10 inches. Then do the math. 10 x [#blocks] = size
EXAMPLE
10 x 5 = 50 inches
10 x 7 = 70 inches
Don't to forget to add seam allowances so your finished size is 10 inches. The size before piecing them together would be 10-1/2 inches. Need it wider or longer, add more blocks.
Hope this helps.