From the Silpat manufacturer's site:
Can I cut the SILPAT?
No. Do not cut the SILPAT as the fiberglass mesh particles might migrate into the food.
Don't do it! They make them in quarter-sheet pan size (about 8.25 x 11.75 inches) if your toaster oven pan is big enough to fit that.
Real Silpats (it's a trademark) are made of fiberglass coated in silicone. You shouldn't cut them them. I have no idea what yours are made of; there's another type of silicone non-stick mat, which is all silicone, without any reinforcement. Those can be cut to size. Silpat makes a number of shapes and sizes, most of which aren't carried by most stores. There's likely to be one to fit your pan.