Measure twice, cut once
Two measurements decide everything. Shoulder height: floor to the top of the shoulders (the withers) while your pet stands naturally. Width: the widest point of the chest or hips. The flap should clear the shoulders by about 2 inches and the width by about 2 inches — a flap your pet has to squeeze through will go unused within a week. This picker starts from your pet's size, then bumps you up a size automatically if the shoulder height needs it.
Door, wall, or sliding glass?
Door-mounted is the simplest weekend project: trace the template, cut, screw in the frame. Wall-mounted needs a tunnel kit through the wall cavity — more work, but it lets you place the door exactly where you want it. Sliding-glass inserts drop into your patio door track with no cutting at all, which makes them the renter-friendly favorite. Pick the install you will actually finish, not the fanciest one.
Security without worry
The honest concern with big flaps is reach-through: a large or X-large opening near a door handle could let someone reach the lock. Fix it three ways — mount the pet door away from the main lock, choose a model with a rigid locking cover for nighttime, and consider electronic doors that only open for your pet's collar tag. For cats and small dogs this is a non-issue; for giant breeds it is worth the extra thought.
Training in a weekend
Most pets learn a flap in two to three short sessions. Prop the flap fully open and lure with treats so going through feels like a game, then lower it halfway, then let it swing freely. Never push your pet through — one scary shove can create weeks of suspicion. End each session on a success, even a small one, and the door trains itself.
