The garage I'm about to do is a second garage near an already existing garage. The supply to garage one is in 6mm and 4mm from garage one to garage 2 (only a run about 10m). 4mm surely enough as it's only going to be 10x 58w lights and 6 double sockets (i'll just stick them on a radial). I've always put 6mm to garages but that's because it's only ever been one garage not a second!