If Permanent Schach is too big, it can either overlap another piece of Schach or stick out of the Sukkah. Should you need to fit it into a tight space, it can be cut down widthways, but not lengthways. However, if your Schach mat is too long, it can just be folded back over itself to double up. For example, if your Permanent Schach roll is 1.6M x 3M, it can be cut to 1.2M x 3M. If the 3M length is too long, it can be folded back over to any length.