Congratulations on an amazing table.
I'm very tempted borrow your idea and make one for our gaming group. How did you decide on the dimensions for the trench and what are they if you don't mind me asking?
Again, top work on the board!!
The board is 3'8" x 4'. For my first board I chose a non-standard size because I didn't want to have to blend the edges to make it a full 6'. 1'10" (8" trench) per half gave me plenty of room to maneuver into the trench as well as have enough Styrofoam underneath to support the thing. I also chose 4' in length so it would more easily fit in my truck or car to travel. A full 6' in length would've limited my storage and transportation options. The current size allows it to balance pretty well at a location with 4'x 4' card tables (such as some of the gaming stores out here).
If you're going to go for something more permanent, I'd probably do a full 4' x 6' (2' per half). That gives you the option of 3' x 3' games or 2 3' x 3' or a single mega 6' game.
The one problem is that Styrofoam comes in sheets of 2' x 4' so you'll have to buy extra sheets and then join them to make the full 6'.
Lowest point at the trench is 1.5" and at the highest 4.5". I used 1.5" thick insulation Styrofoam sheets.