What are the size restrictions of a Clone (from spell)?

Some other players of 5e D&D have suggested that the Clone spell only works with a Medium size body. Putting this into perspective (and putting aside obvious yet possibly reasonable discrimination against creatures such as gnomes, kobolds and halflings), what is the actual RAW on this?

Question: What living creatures can be cloned by the 8th level Clone spell – from what micro-minimum to what absolute maximum in size could this thing be?