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I'm sure the list is out there somewhere but i'm flucked if i can find it!
So far i have -

Avebury 300m
Stanton Drew 113m
Long Meg 107m
Ring of Brodgar 103.6

The Waun Maun circle is dipping in at 110m but i'm discounting that as there's little evidence for it bar MPP and his findings. Though the Ness of Brodgar website seems to kinda support it's place in the list.

https://www.nessofbrodgar.co.uk/ring-of-brodgar-relegated-to-fifth-place-in-stone-circle-size-table/

Maybe Stannon gets in the top 10? It's years since I've been there but I remember thinking it was pretty big.

Twelve Apostles at Holywood just north of Dumfries. Pretty big.

Not even close to the big ones, but this is a handy guide to size of Dartmoor circles.

http://www.dartmoorwalks.org.uk/resource/circles.php

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.

Shame it's not henges - Figgsbury Ring is up to 187m long by 151m wide.

And it's lovely.