I would say that 90% of the time on BA day flights, pyjamas are offered in First. If not I ask for them. BA used to do XL in PJs which was better and in fact the PJs in First over the years have been value engineered or enhanced. they are now much poor quality. When they were first brought out they came out in a nice pack with a label inside which read "Wash before use" which always made me chuckle. They were of such a quality that I always kept them, nowadays I don't bother as they are sub Primark quality. By comparison, the Pjs in Singapore Suites are made by Givenchy and are well worth keeping.
I am 6ft 3 and 15 stone and the L size is really not big enough.