It's not just about size, its so much more. Ergonomics, column stagger, ortholinear etc. There are tons of different layouts and shapes and sizes, splits, non splits etc etc.
I build these keyboards from scrtach. I source the parts myself and put them together
Here are some of my keyboards, there is the corne on the top, a treadstone48 in the center and a M60 in the bottom right ...