What is the actual hole opening supposed to be for a rectangular Gable Louver if, for example, the vent is 18" x 24"?

Here is some guidance for cutting the hole for a Gable Louver.

The installer should cut the hole to the approximate size of the actual vent while leaving a surface for fastening. So, for example, the 18" x 24" gable louver would have an approximate opening of 17" x 23" roughly.