What should I do about a 72 inch door?
Depending on the door construction and your own comfort level, you might be able to modify the door you have by cutting an 8" section out of each vertical stile in the top or bottom of the door, somehow attaching them back together, and then reinstalling the screen in the smaller opening.

Given how simple a screen door is though, and especially on an outdoor shed where things don't need to be perfect or ornate, I'd recommend returning the door you bought and just building your own.  You can make the frame out of a double thickness of standard 1x4s or 1x6s, laminated with glue and brads.  If you lap each corner you'll end up with a frame that's quite strong.

For the screen, I'd use a mounting system like Screen Tight; I believe it's sold at both Lowe's and Home Depot.  It works great and isn't too expensive.