Vertical space is. You can use pretty much as much horizontal space as you like.
Partly it’s down to personal style. Do you set up a patch, dial in each pot, then hit record, play the keyboard, and stop the recording? Or do you do like Sam does, using the pots as performance controls, manipulating them throughout? In the latter case it’s more important to have large enough pots and large enough spacing to facilitate grabbing and turning them without bumping into other knobs. (Hence Kosmo.)
I like distinguishing inputs from outputs, in my designs panels are white with black silkscreen, but outputs have black boxes around them with white labels. Some people like colored nuts to distinguish different types of inputs and outputs. Not my thing. I do usually like to keep jacks separated from pots, again to minimize interference when reaching for a knob. Generally I put rows of jacks across the bottom of the panel with knobs and switches above that. I don’t usually have much graphics aside from text labels (or sometimes icons, like for wave shapes on a VCO, but mostly I prefer text) and lines, but then again I’m not much of a design artist.
Well, there’s this:
Then again there’s also this: