@medeek said:
What is the typical header thickness between the top of the vent hole and the top of foundation?
What I usually do is provide a solid wood header within the floor framing and suck the vents up tight to the top of the stem wall.
As to size, it will vary depending on the topography of the site, total size of the underfloor area, etc. Also, here in California if you are in the "SRA" (State Responsibility Area, which is where fire protection is provided by state forces such as Cal Fire) you have to use approved ember-resistant vents which are further limiting.