Assuming the theory is correct, that it is a complete circle, doesn't 25 stones in a circle of diameter 10m seem claustrophobic? Unless the stones were low-sized, I suppose, or narrow.
It's a very similar size to many multiple ASCs which would have had 11 or 13 stones for a roughly equivalent circumference. Drombeg (17) and Carrigagrenane (19) excepted.
He does say that it's a max figure, but what he was looking at would have determined his estimate.