What size squares would you make for a twin-sized rag quilt?++?

I am no good with math..I made a rag quilt a couple summers ago using nine inch blocks but with seam allowances and i don't know somehow it was too small. I want to make another one this summer and make sure to get the size right. The quilt should be 90" by 60" I think. So if each row has five squares (it is a checkered pattern), what should the size of each square be? Thanks in advance. Any help and tips are appreciated :)