Although I'd suggest editing the Pathing - Collision Size, that's unfortunately only in the unit section of the object editor (as I've seen so far.) Maybe you can try editing the Pathing - Pathing Texture on the destructibles part. Find something smaller and adjust accordingly to the size you want. Or, though I'm unsure if it will make a difference, you could try Art - Occlusion Height, to see if it spawns any results.
An idea for a workaround however would be to create a new building, remove any abilities and change the model to the gate variation you want. Then you can edit the size and collision to what you want so it's walkable as you need it to be. Then trigger it when necessary for it to be invisible and have no clipping, replace it with the destructible that is the open gate for units to walk through, and when done, simply repeat the process in reverse.
Or perhaps you could try having a building variation. I'm not sure if it can be accomplished, but maybe you could add 'Alternate' to the animation and it will show the opened gate. Then apply no collision or no clipping via triggers, and voila, a working gate that you can scale as necessary. Oh, and don't forget to remove the selection size for ingame, otherwise you'll click on the opened version if that can be accomplished.
Hope any of this helps,
-Vikus