Pushing this as deeply into ad absurdum and slippery slope territory as possible:
The smallest possible limit must be for the basic RAW Clone spell should be: any creature with less 'flesh' than a cubic inch, i.e. the amount required for the base-component to cast the spell. This requirement could be vetoed by any creature able to cast this without use of material components, such as the 5e D&D spell-casting dragon variant of CR24+. Such component-free castings would be able to duplicate one bacteria cell or possibly a virus... were they considered a life-form.
The largest size possible aught to include ANY living creature, as per RAW / spell description. Thus, the 'Garden of Eve with teeth and claws' planet for the Na'vi, Pandora seems both alive (and even sentient). Granted, it may be tough to find a crystal container big enough, but once again, casters not requiring components can even do this casting quite easily. Those using Wish spell can create duplicate living planets as a move action, once a day, without threat of exhaustion or other such reprimand-repercussion.
'Look ma, a new planet just came into orbit!' This would also be ideal if one had something like a living plane of existence or even living magic items. If a magic item is made from a fae, fiend, celestial or elemental, are they alive? Would such a Clone rebuild this magic item as a 'body'? This would be stuff for other questions and Exchanges of Stack.
Lore wise, this would explain why the Abyss is notoriously infinite: someone figured out how to replicate living planes for their homestead. It may be that the Abyss is a cloned wasteland created by one otherwise-bored ancient dragon, casting this as many times as he or she has in charisma modifier points.