The manual says Masks are resized to 320x200 even for 640x400 games. Did I read that incorrectly?
I sure did. It says: NOTE: For hotspots and walkable areas, the mask must be the same size as the background, but at 320x200 resolution. So, if you had a normal sized 640x400 room, the mask should be 320x200. For walk-behind areas, the mask is the exact same size as the background image.
Correction: Walk-behind areas are the same size, but Walkable Areas are not, so be careful to give the proper border so your characters feet don't seem to walk 'under' the fence (and ground). Specially by the last chained area...