The traditional starter railroad is on a 4x8 sheet of plywood. However, a rectangle has limitations - you are pretty much limited to a little oval of track, and it is difficult to use the corner space in a practical way. Here's an easy improvement on the good old 4x8.
Cut off two 24" x 24" triangles, outlined above, off one end.
Flip them around and attach them to the other end. Now your 4x8 is 4x10, with a shape better suited to a big oval, with a longer mainline run. Formerly wasted corner space is gone.