60-70% of a 4x4 at the minimum. Probably shouldn't have more than 6-8" of straight line open space.

Once the large pieces are done I'll start on smaller pieces. I already have a bunch of wall molding cut up for police style barricades, and some soda lids I'll be turning into barrels. I've a got bunch more PVC fittings from a project years ago so I'll make some more of the industrial pipe stuff and I'll do something with the 4-5 plastic electrical boxes I have as well.

What I really want to do is a modular space ship or underground table, where the square footage is the same but it's all made up of connecting corridors, themed rooms, and scatter stuff.

You could even do a complete interior table, where the skirmish is taking place inside a single building.