Here are the key points to consider in purchasing a sliding glass pet door
Don't just assume that 'it's standard' as they do vary. Using a ruler, measure inside the track, very top to very bottom. Always compare your track height to the adjustment ranges shown for the various pet doors.
Determine how wide the flap needs to be
Open your sliding glass door wide enough for your pet to come through comfortably. Allow room for growth as needed. There's your minimum width. Don't buy a flap that isn't at least that wide.
Determine the flap height and step-over dimensions you need
Ideally, the top of the flap will be at least as tall as the top of the pet at his 'withers' (top of his shoulders). That way he doesn't have to crouch to use the pet door. The 'step-over' dimension is the height of the bottom of the flap above the base of the pet door. That's how much he has to step over.
So, flap height plus step-over is the height of the top of the flap.
Generally, the lower the step-over tthe better. Especially when your pet is older or arthritic. So you might prefer a flap 17" tall with a 3" step-over to one with a 15" height and 5" step-over. Both reach 20" at the top, but the taller flap yields a lower step-over.
More flap and less step-over can also be helpful if you've got a small pet that needs to share with a larger one.
A pet door that incorporates a user adjustable step-over makes it simpler and offers more flexibility for the future. These are only available with Patio Pacific pet panels.
Check the color of your framing
Most brands are available in white, satin (aluminum color) or bronze (very dark brown). You can paint any of these to get a different color if you wish but the easiest to paint is the white. Check with your local paint shop for primer and paint recommendations.
See if you have dual-pane glass
You probably do and, unless you're in a Very temperate climate area, you'll want to choose a pet door that has dual-pane glass also. You can tell by placing a finger on both sides of your glass. If they look like they're touching, you have single pane. If they stop about 1/2" apart, you have dual pane.
If you want dual-pane glass, how about Low "E" also?
This coating on the inside of dual pane glass units performs much better than regular glass in insulating your home.
Are you looking for high wind resistance--so you want higher magnet strength on the flap--or are you planning to share a larger flap between a big dog and a smaller dog or cat--so you may need less resistance, especially at the bottom of the flap?
A Variable Magnet Strength Flap will give you enormous flexibility from extremely wind resistant to very easy to push and everywhere in between. Look for this feature if you think you may have a need. Pet
See if you have vinyl frames
There is more standardization in the newer vinyl framed panel doors. Some patio pet doors are manufactured exclusively for this application.
Worried about bugs flying in between the patio pet door and the screen?
Check out the Bug Warden. If you ever keep your sliding glass door open for ventilation, this is a 'must' product.