uh...
well Yeah I think collision size doen't, but try anyways, and what he means is turn the movement type of the unit from foot to flying, that just changes what he obseveves as an obstacle, a unit with movement type flying has nothing except other flying units as obstacles, so the unit with movement type flying will be able to pass thru walls, blizzard uses this in combination along with height to make it look like it's flying, with no height he will walk thru