Generally most standard barn doors are approximately 36 x 84 or 42 x 80 with enough extra lip to overlap most standard door frames 36 x 80 by an inch or so.
Barn door height.
Barn door measuring guide for door openings with casing molding.
The graphic below simulated a door over the opening.
Measure across for the width and from the top of the casing molding to the floor for height.
Always remember that your barn door railing system should be at least double the width of your door frame possibly longer if you have an odd shaped door.
For the door height we recommend measuring from the floor to the top of the casing.
72 would probably be even better.
Most barn door sizes fall anywhere between 36 wide on the shorter side by 96 tall on the longer side.
Add 1 to 2 per door for overlap depending on privacy needs.