The standard size for a UK Kingsize mattress is 150 x 200 cm / 5 ft x 6ft 6in. If you have the space it is often difficult to decide between buying a Double bed or mattress or a Kingsize bed or mattress. On The Sleep Council’s website it suggests that the bed should be 6in longer than the tallest person and allow both to lie side by side, hands behind heads, without elbows touching. Whatever you decide there is a good range of Kingsize mattresses from retailers including Argos, John Lewis and Tesco. Popular brands include Simmons, Silentnight, Sealy and Pure.