I think to best understand sizing adjustments, you need to have a really good handle on what makes a bike feel like “this bike” and not “that bike”. If you can take a set of parameters (trail, reach/stack, CS length, tube choice for example, or however you would normally define your design for a given size) and say “I expect this bike to function with X, Y and Z characteristics” then it should pretty easy to extrapolate to bigger and smaller sizes. I guess the hard parts are a) knowing how to do that in the first place, and b) knowing what proportions to aim for with a given rider body geometry.
Find a bunch of mass-production bikes that are similar to what you’re working on and look at their geometry charts - you probably shouldn’t copy directly but you can get a good idea of where to start. Also look at ratios between given parameters, eg trail:wheelbase (hat tip: @BS_Industries post in the 14" kids bike thread).