I'm not great at math but I do know that nine 2 1/2" x 2 1/2" squares will make a 6 1/2" unfinished block (6" finished block).
If you make 88 9-patch block this size, you could made a quilt that is 48" x 66" by using 8 blocks across & 11 blocks down. You could enlarge the quilt width slightly if it needs to be 50" x 65" by putting on a border around it. Or you could make 99 blocks & put 9 across the top & 11 blocks down & that would make your quilt 54" x 66".
Now please wait before you start making all those blocks & see if someone else comes along & lets me know if I did the math right!!