Sckor, making a location bigger actually doesn't make it any more likely that a unit is placeable at the location.
If the center of the location is on the edge of a cliff or on water or some other illegal terrain, then a ground unit will be unplaceable no matter how big the location is.
If the center of the location is on legal ground terrain, then a ground unit will be created at the center of the location, or (if there are units in the way) as close to it as possible, up to some maximum distance (at which point the unit becomes unplaceable again). Again, the location size doesn't make a difference.
Air units will only be affected by the latter restriction.
If there are ground units at the center of a location, a building cannot be created at that location.