The 'golden ratio' for small boxes is height times 1.618 for the side, times 1.618 for the length. If making a box that's for no particular purpose I'll tend to stick close to that but am happy to change things to accommodate the stock I have available. It's a shame you haven't done the basic set up on the Leigh D4R as once you've done that you can get some boxes made pretty quickly. It's much quicker and less fiddly than the Incra fence system.