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Is it possible to create a massive invisible collision? I'm talking huge here, not very tall but very very wide.

I'm seeking an alternative to keeping flyers out of my main infantry combat area (right now I use mountains and a flyer height limit, but this is no fun for the rest of the map)

You could always rescale the shipped ones.

He could but that scales them in all directions.
What you could do in XSI is make a very small mesh (like a tiny little square) and give it a very big primitive collision in the rectangular shape you desire. And if you want it only to block vehicles (this would apply to flyers and land based) name the primitive something like this p_-v-bigcube. That way you would still be able to run over the mountain.
This is of course if you are talking about player flown flyers as opposed to AI.
If it is just AI you want to keep out that can be done with barriers in ZE