Medium-ist end of larger UK sites.
Beltany stone circle 45m
Gamelands Penrith - 44.5 by 37.5 metres
Fernacre 46m by 43m
Craddock Moor 39m
Girdle Stanes 38m
Arbor Low 42m by 37m
Ballynoe stone circle 35m

France
Lacam de Peyrarines 120m

Slightly off topic (as ever) and I'm sure you're aware, but the Priddy Henge Circles are approx 152m and the Thorbourugh Henges Circles are approx 250m. Durrington Walls is 470m.