[...]on occasion the queen has hosted at Windsor. The last time was in 2006 it seems. Windsor is the largest of the queen's homes, and certainly has more guest space then Norfolk. It does lack the privacy of being their own home though. If Charles plans to slim down events when he is king, and William and his family have Amner, the lack of space at Sandringham will be little worry.
Or he may choose to switch Christmas to Balmoral. We know that he and Camilla love their home up near Balmoral, they spend new years there.