I think you need to decide on a size first, then start tinkering with track layouts to accomplish what you want to do. In a 12x12 layout, you should be able to have a couple of O72 (or preferably greater) dual tracks to allow you to run your big stuff. That still leaves a lot of area to work with inside.
Depending on the space, you can get wider curves with an around-the-room format vs. a table format. If you do a 12x12 table, consider having the center open with a lift bridge to access the inside. Remember, you're going to have to reach stuff anywhere on the table, consider that in the planning.