Measuring for patio screen doors is pretty easy
Basically for the width you measure the glass sliding door and add a half to a full inch.
For the height, you measure half way in the deep channel on the top
to the top of the track on the bottom.
You might get a small latter and put it in the middle of the door and go up the latter and take your tape and put it inside the channel and measure how much it goes in then bring it to the outside of the channel and put a half way mark on the outside of the top track, you do this in the middle of patio door set because the top header sag's in the middle so you must measure to the lowest point of so the screen door wont get bogged down in the middle
Then get off of the latter and pull it away, then pull the tape out of the tape measure case and put it on the top of the track and measure to your mark, that's the measurement for the height of the new screen door.