Since the Saw board becae popular,here is a router board for those who dont have a router table.
Basically its a frame made out of 1"x1" wood and two pieces of plywood glued and pinned to the underneath of the frame with a 1.5" gap between the two boards to allow the routers cutter to glide along the work area to be cut. The width of the inner frame depends on your routers base ie some routers are circular and some are 3/4 circular with a flat edge,I have a cheap router that has two flat edges so whatever your routers base is take this into consideration if making this router guide board.
A crude drawing of the router is in place in the pic to show what I mean,the sides of the base touch the walls of the inner frame. It'll give you clean straight cuts,rebates if constructed with precise squareness of the board.
Length is down to how big the individual wants it.