The most important aspect of your generator house is the floor you use.

We parked one of our tractors in a pole barn with a concrete floor and the other tractor was in another barn with a gravel floor.
The tractor on the gravel floor had started to show signs of rust within months whereas the tractor on the concrete floor was rust free.

There was plenty of ventilation and the ceiling is about 9 feet high in the barn,
but the moisture in the ground permeated through the gravel floor causing the rust.

If you use concrete for the floor make sure it has a vapor barrier. If you use wood make sure its at least 8" off the finished grade.