One, I would not take Mavis's advice, unless you want feathers and down everywhere.
It will not hurt a duvet to be in a cover that is too small. It will be a bit lumpy at the bottom. But if it were me, I would make the cover longer to accommodate the duvet. Just add a band of fabric to the top. Far earlier than adding to the bottom, although you could put a band of colour anywhere on the duvet.