Shelf size
Front opening is what a jar has to pass through: between the front legs, under the rim.
Shelf
The rim does two jobs: it stops the shelf sagging (a deeper rim is far stiffer than a thicker shelf, for less plastic), and it's what sits on each leg's ledge, with the tenons that lock into the legs underneath it — so a deeper rim is also a sturdier joint. It costs that much front-opening height.
Legs
Chaining
Each tenon pocket has two small ribs that squash as you press the shelf down, so it stays put without pins. Tighter than you'd like: lower Press fit. Loose: raise it. 0 turns the ribs off.
Printing
PETG suggested: kitchen cupboards near a cooker or kettle get warm, and PETG holds its shape under a constant load of jars where PLA slowly sags. It's also tougher, so the crush ribs squash rather than crack. PLA is fine for a cool cupboard with light loads, and it's stiffer, so a PLA shelf bends less. If you print in PLA, start with a lower Press fit.