The bigger the better.
In general, vets recommend that the litter box size is 1.5 times the length of your cat (from tip of nose to base of tail). The litter box needs to have sufficient space for your catto to move around, dig and cover without feeling restrictive.
Don't buy a litter box that just looks good to fit a space in your home; consider your catto's needs first. Your catto shouldn’t have to become a contortionist for each visit to the toilet.
A litter box that’s too small becomes a source of stress for your catto. If your catto becomes uncomfortable in the box, it may choose another location and it may not be to your liking.