All brands of sliding glass dog doors shown below offer optional shorty patio pet door panels that are under 77 inches tall.
If you have an extra short sliding door the following brands will customize a sliding glass dog door to the height you specify:
Thermo Panel IIIe - made by Patio Pacific for double-pane sliders
Quick Panel III - made by Patio Pacific for single pane sliders
Hale Standard Patio Pet Doors - not compatible with Andersen or Pella doors.
Hale OMNI Patio Pet Doors - will work with Andersen or Pella vinyl frame doors.