Doesn't mean for a couple, and shouldn't mean just a double bed.
It should mean a room at least three times the area of a double bed. I think that's a good definition.
However you'll find a lot of people defining it has having a double bed in it, meaning the rest of the room is as roomy as a charter flight to Malaga in August.
This is one of those habits that has crept in from that notorious profession: estate agency from whom we have got 'Islington' to mean the borough of Islington (which stretches from Hackney to Clerkenwell) and rents quoted per week instead of per month. None of this is helpful at all.
So beware the definition of double room : you will seldom see a room described as 'single' these days. Also 'double' beds come in a variety of widths from 4ft (120cm) to King Size double bed.
By the way its also kind of worth enquiring about the quality of the bed, or finding out for yourself by sitting on it.
Talking of dounble beds and people coming to stay ... it's worth enquiring about boyfriends/girlfriends staying over and that the flat or house attitude to this is.
'O' is for orthopaedic, 'B' is for bouncy castle.