Sizing a Litter Box for the Cat You Actually Have
Most litter box complaints are size complaints wearing a disguise.
The length rule
A box should be roughly one and a half times the length of the cat, nose to tail base. Most commercial boxes are sized for a small adult and are cramped for anything larger.
Entry height
Kittens and older cats with stiff joints need a low entry on at least one side. A high-sided box with a cut-down entry gives you both.
More boxes than cats
The standard guidance is one box per cat plus one. It is more about avoiding queueing and territory than about capacity.