updated by @chocotenango: 06/07/15 03:02:34PM
Generally, the larger the nozzle number, the wider the pattern. But nozzle hole size can also be matched the the viscosity (thickness) of your materials being sprayed. Thicker viscosity will need larger nozzles.
In general, if you are working with 50% cocoa butter 50% chocolate mix at 29C-34C using a standard compressor on half power, then a medium nozzle should give you a nice mix of fine and wide.