I started with a simple double track loop (double track - I have two boys!). It was simply first and second radius Hornby curves, with a set track crossover. The tracks are 67mm apart, (as per the Hornby standard) and the straight section is 2*167.5 = 335mm long - enough to accomodate the crossover. On the plan below, the dotted line is the track centre. Each board is 2*67 = 134 mm wide (actually cut slightly less to allow adjacent boards to be added (see later).