I'm more than 240 lbs and the sleeper suits fit me just fine. Of course that is assuming you can get one of them... on two recent 'day flights' I have been initially denied a sleeper-suit, on a trip to New York they eventually relented after lying that there were none on the plane - then another FA giving one to Sim!
On another trip I was forced to unpack my luggage and extract a pair of shorts and a Rugby Shirt.
If anyone from BA is listening to this, I would like to say it is bloody discusting, we pay a fortune for those tickets and the price doesn't change if it is day or night, so why should the service. You shouldn't put your cabin crew in a position where they become the brunt of passenger complaints, and you shouldn't advertise that you provide sleeper-suits when half the time you don't and finally it is up to me to decide if I want to sleep or not, you shouldn't be so presumptious as to decide if I am going to sleep or not.
The fact that at a time when other carriers are improving their premium services(mentioning no names), BA don't seem to have learnt the lessons from their earlier attempts to dumb-down their First service.