This is really for Scottish TMAers...

Well googling because of ignorance, mine, ( though they are to be found only in Scotland and Ireland?) it turned into an interesting read, they are part of a stone circle, the 'altar' from which you can see the moon, would form part of the dimensions of the s/c. So for a start, 'Old Keig' ....

'At Old Keig a recumbent stone circle situated on a slight crest on a ridge, and within a narrow windbelt. A recumbent stone, 16ft long on top, 6ft thick, 6 3/4ft high, two flankers, the westerly 5 1/4ft above the turf, the easterly 9 1/2ft, and a third orthostat, 9 1/2ft high, to the east, survive. Five other earth fast stones in the vicinity look as if they have been disturbed.'

https://canmore.org.uk/site/17530/old-keig

http://www.themodernantiquarian.com/site/191/old_keig.html

As well as RSC, there are single recumbents in various places, but often there's uncertainty about whether they are anything other than natural. With no other stones needed, when does a big lying-down stone become a prehistoric monument?

Eg http://www.themodernantiquarian.com/site/13035/cefnyrhenriw_recumbent_stone.html

The four broken pieces of the recumbent Er Grah megalith in Brittany, “...is the largest known single block of stone to have been transported and erected by Neolithic people.” (Wiki). It measures 20.60 metres (67.6 ft), with a weight of 330 tonnes. Being in France, though, that doesn’t count :-)