As noted on pages 27-28 of the manual, Type menu should work and allow choice. In my 2.2.0.0 this does not work??
Nowhere in the manual there's any indication that changing the parking TYPE would have any effect on its SIZE. In fact, it just confirms what I've said in my previous reply, that changing the type it's usually done to set a non-cargo parking which was set as cargo by the scenery developer, and vice-versa, in order not to confuse GSX with the airline assignments.
The parking type is one thing, and the size is an entirely different setting. You can have a parking with 5.0 meters radius set as Heavy, or a 60.0 meters radius set as "Small", this is how the .BGL file format works.
Maybe, you assumed that, since some scenery editors like ADE or AFX, when you create a new parking, automatically set a default size, but that's just an editing convenience used by these programs, but GSX keeps them separate, which is how the underlying .BGL works and, it's better this way, because if we changed the radius automatically, the default vehicle placement would change too, so it might gets confusing, when the reason to change the type was just to fix 3rd party sceneries that flagged the type in non-standard ways.