Is possible to create a super load door made of a single long long box? I want to use it as a sort of transition area like the autoload doors ....

I imagine it would be possible, but the player would not necessarily arrive on the other side at the same place them left if it's that big.

You would be better of scripting it.

No is fine if arrives with a marker .... I just need a supersized door ...

OnTriggerEnter
game.fasttravel

you can make the triggerbox as large as you can fit (usually needs to be an esm for the script to work properly, but with the size of your mod, you are probably going to use an esm anyway i imagine)