My old fan is very undersized for the size of the bathroom, it should be about 110 cfm and the one that's there is 40 or 50. This leads to way too much humidity especially in the winter, with lots of condensation on the windows. The existing g fan has about an 8 inch square cutout, the 100 cfm + fans are all larger.
Dave in WI
"Daddy, it's five o'clock sometime!"