Measure the bottom of the door and add a couple of inches to each end to get your length. it should be about 3 inches tall depending on how big the gap is under the door. It needs to have some method of weighting it so it stays in place like sand, walnut shells, beans etc. When I was small we used to have a long tube with ears/eyes/nose on one end and a tail at the other for a wiener dog draft stopper.