This isnt so much about moding as it is more about the engine itself... but perhaps the information could be useful...

Anyhoo, simple question... is there a max height in an exterior cell, and is there an invisible wall there? Ive made some personal mods with things WAY up in the sky and have yet to find any such wall, but i got this guy insisting there is one.

Everything have a max limit on computers, so it's there is a limit. But I have no idea what it is, although I would guess on 2,147,483,647 units (signed 32-bit integer), and seeing Megaton is ~5,000 units wide, that's quite a long distance.

The Havok limit is 30,000 units in any direction (X, Y, Z) before you'll start getting errors. I agree with you that it seems quite high when your outdoors!

Miax

now to find out for sure if there is a wall...

I do not think there is a wall, but if you exceed the limit you should get warnings and errors in the geck.

My guess is that if you go high enough in-game, your z position will become negative once it passes the limit, but I don't know if you'll be able to reach the limit. There's not much to climb on, and http://geck.gamesas.com/index.php/SetPos probably won't put you above the limit.

Freddo's guess value sounds like a good one to me.

Cipscis

http://i19.photobucket.com/albums/b200/staplehead78/Fallout32010-04-0517-22-58-46.png

9million units, and some guy is saying there is a wall if you blow yourself up with enough bombs... though thats apparently the havok limit.

Someone needs to make a mod like "Tower to the Moon" and have platforms outside of the tower every million units. I wonder how long it takes to fall when your that high?

Ha! Like http://www.youtube.com/watch?v=cQAHrEkWnr0. I'd Imagine you could ~tcl and check. In Oblivion, while riding a dragon mount or magic carpet, I never hit an invisi-barrier.