Cut a hole that is the size of the flap dimension in a piece of cardboard, stick it in a doorway and call your pet thru to make sure that it is comfortable. If you are considering a panel pet door, cut the hole to simulate the rise, e.g. if you are thinking of a 5" rise, cut the hole 5" from the bottom of the cardboard.
Step 1: Find out your minimum flap width. The easiest way to do this is to open a door just wide enough that your pet can get thru, and then measure the width of that opening. This is your minimum flap width, you can add an inch to make things more comfortable.
Step 2: Find out your pet's height. Measure your pet to the top of his/her back at the front shoulder. The top of the cat or dog door should be at or slightly above this height.
Step 3: Find out your pet's clearance height. Take a measurement from the floor to your pet's chest. This is the clearance, and your at or dog should start at or slightly below this measurement. On the panels the distance from the floor to the bottom of the flap is called the rise measurement.