@JeremyECrawford When casting Resurrection, how much of a body do you need to count it as a 'creature'?
— Thomas Wojcik (@LolomirNL) September 5, 2017
Resurrection restores any missing body parts, so it doesn't need much of a corpse to work. The exact amount is up to the DM. #DnD https://t.co/C2Y3XE7aac
— Jeremy Crawford (@JeremyECrawford) September 6, 2017
Presumably even a small piece — the equivalent of an ear, or even just the finger tip — would be sufficient. (To cross-compare with similar magical effects from some legends/myths, even a lock/few strands of hair — especially if having a small follicle still attached — was the “material component” for restoring to bodily life certain beings.) So, really not much. The distinction between Resurrection and True Resurrection is that the body may be properly destroyed (cremated — not just burned at the stake, but properly cremated and likely scattered at sea, et al) in the latter case and it may be restored in-full with TR vs. R. (Examples from shows where similar has happened: R = the 10th Doctor from Doctor Who growing a whole body/incarnation from his previous regeneration’s severed hand; TR = when Cordelia from American Horror Story: Apocalypse restored Myrtle from literally nothing — no body, proper, just the memory of her — after being dead for several years.)