Jan, I'm lousy at math, but if you want the largest squares from a fat quarter you can theoretically get one 18"x18" or 4 that measure 9"x9", but since not all fat quarters are exactly the same, you could just fold yours into quarters and see how large you can cut them!
If you cut each FQ into a large (18") square you could use 9 of them to make a quilt about 54.5"x54.5".
If you cut each FQ into 4 (9") squares you could use 36 of them to make a quilt about 53"x53".