Normal size vent I had to trim just a bout half inch from one side to fit normal size vent
No fiberglass. They are polyester. Reminds me of a stiffer version of quilt batt fabric or inside insulation of jackets. It’s sheets of firm white filter that can be cut, and fitted inside metal floor register vents.
They go under your hvac vents in your floor. Adding vent filters to your home serves as an extra line of defense against dust and allergens.