If you don't live in the countryside, this is undoubtedly the most important object of a cat's kit, you shouldn't choose it too small, in fact, the cat, growing up, could consider it too small and therefore do its needs outside (which would be unpleasant).
It's best to avoid regular sand and sawdust (they could give off a bad smell). Closed litter boxes with internal odor-absorbing devices are also recommended, to be changed every three months.