How big is the Settlers of Catan board?
Roughly 15x16" for the base game.

roughly 15x20" for the 5-6p expansion.

About 17x28" for framed base game with C&amp;K, and another 3.125" for 5-6p.

Seafarers can be considerably larger... up to 24x36"

I've not bothered with Traders and Barbarians, so I cannot speak to it.

Note that the travel version (which is base game only) fits nicely on an airline tray table.