I got some huge 5ft MDF boards which are about 3 ft wide, got three I line up on top of a table, making it easy to take apart and move round, its very think stuff that won't bend off the top of my head its about inch and half maybe..thick anyway's
I am no expert though just starting doing this, I just guessed this would be the best material, and well it is,
probably more 5x2 but they make a nice rectangle when lined up just measure your planned track size area along with the table top you thinking of using you should be able to come up with something that fits both, if you plan on a huge layout track you might need a table with a bigger span to stop the mdf tipping every time you lean on it