How to change WidthInGameUnits

[WHAM]

Okay, maybe I've got something mixed up here but:

When I set the game resolution to 640x400, I thought it also meant I could set walkable areas and such in 640x400, but
instead the room only has 320x200 as its "WidthInGameUnits".

This is also a problem when I try to give objects and characters accurate locations.

What does this mean and how do I change this attribute?

http://www.whamgames.com/images/Forumsht/waaah.png