The map shows lots of hut circles here, so initially I wondered if the circle must have been one of those. But it sounds far too grand for that.
CWM MAWR. -- Distance 3 miles from Dolbenmaen, in the way to it, several pillars of difference appearances, &c.; none of them equal to those of the grand monument, whose situation in upon the gradual slope of a very high hill, commanding a most extensive prospect, viz. the whole Isle of Anglesey, part of Ireland, &c. The first object in approaching it is a lonely pillar, distant 160 paces from the grand Ellipsis. this colonade is in diameter, one way, 44 cubits, the other, 36; consisting of 38 upright stones of various forms, heights & sizes, as well as distances from each other; some turgescent, some flat, some incline one way & some another; some are pyramids & some are cones.
The vulgar believe that no one can count them. The area of the monument violated by the plough & harrow, &c. Tradition says that upon one of them being carried away to the adjoining farm house, for a lintel over the door, such a dreadful storm of thunder & lightning ensued, that the sacrilegious hands were forced to return it to its former place. However, the author says that the vacancies show that several have been carried away, &c.
From Archaeologia Cambrensis v14, 1849. A cubit, should you be too far indoctrinated into the metric system to know, is the distance from your elbow to your hand. Or fingers. &c. The information (hence the mention of the 'author') is taken from a manuscript dated 1772.