This new member would be useful for scripters for various reasons: mounted characters could be scripted to became incapable of riding inside some houses, short and tall people could have their own advantages and disadvantages (fighting is easier if you are taller, LOS for actions, ...). Also, using spells like polymorph could change, not only the way you look like, but also the places you can get to (think small rat versus human).
Member should be r/w so whole potential could be used, static value (in config files) wouldn't really do that. Height should be mobile's member because that way NPC:s could be given their own benefits in combat and other things.
What this would probably need is serious modification of core's functions, way the LOS and pathfinding works and are checked... I suppose that right now most if not all mobiles are sharing the same (hardcoded?) height and that would need to be changed, too. Of course all that would take some time and effort needed elsewhere. I still feel this should be implemented because of all the special features we could create. Core's support is needed because thing like this is hardly to impossible to create with just scripts.
