I wonder if those saying a double is plenty of room are talking about what others are calling a king. Depending where you look a 5ft bed (king) can be called a double.
It happens on every thread bed size is talked about!!
For the majority of people in UK and Ireland it's:
Cot (23" x 47") 60cm x 120cm
Toddler (2'3" x 4'6") 70cm x 140cm
Small single (2.5' x 6'3") 75xm x 190cm
Single (3' x 6'3) 90cm x 190cm
Euro single (3' x 6'6) 90cm x 200cm
Large single (3'6" x 6'3") 75cm x 190cm
Small double also "Prince" (4'0" x 6'3") 120cm x 190cm
Double also "Queen" (4'6 x 6'3) 135cm x 190cm
King (5' x 6'6) 150cm x 200cm
Super King (6' x 6'6) 180cm x 200cm
When it comes to the sizes double and above some people refer to a King as a double (or Queen), the Super King as King. Australia and America use these names (and other counties too)
Also I think in American bed sizes might have similar names but the actual measurements differ too.
There's bigger sizes too - Emperor, Caeser etc! Personally I'd opt for the biggest bed i can fit into the room. It's only when you get to Super King do you have the same space as you would in a single bed.