I have been doing some research into this, so that I can add (eventually) seat width info to the
A quick guide to BA partner long haul business class products thread.
Here is the info collected so far. Bed mode measurements take into account retractable arm rests.
Aer Lingus
A332, A333, B752: 19.8" wide (19.8" bed mode)
American Airlines
B763: 19.8" wide (19.8" bed mode)
B772, B788 (Concept D): 20.8" wide
B772, B789 (Super Diamond): 21" wide
B773: 20.5" wide (25" bed mode)
A332, A333: 20.5" wide
British Airways
B744, B772, B773, B788, B789, A380: 20" wide (25" bed mode)
A321: 19.8" wide (19.8" bed mode)
A318: 20" wide (21.5" bed mode)
Cathay Pacific
A333, B772, B773: 20.5" wide (25" bed mode)
A350: 20.2" wide
Finnair
A332, A333: 19.8" wide (19.8" bed mode)
A359: 21" wide
Iberia
A332, A333, A346: 21" wide (21" bed mode)