Single mattresses at 92cm wide and 188cm long provide the basic package and suit young children or, at a push, an adult. They are easy to find bedding for and provide the perfect accompaniment to a single bed, which is an economical option both in terms of budget and space.
If you want to lie down with your child to help them get off to sleep it will accommodate, but adults should consider a single XL which is 203cm long.
If you want flexibility from a spare room you can use two single beds with a super king size mattress for couples, and divide them to be used with single mattresses if you have children to stay. This is a good option for grandparents who look after their grandchildren but don't want to give up the option of hosting adults. A single mattress works well for day beds, bunk beds and small sleepouts.