There are many requirements for a space to be Van-accessible. The first is that it should be 96" wide and you should have 98" head clearance. You should also have no more than a 5% slope from the handicap space to the building main entrance and no more than a 2% cross-slope along that route to the entrance.
If you do a Google search for "van accessible parking", the first result from ada.org gives more specific info.