I would never choose to sleep in a single bed. Always my knees or bottom sticking out - and never being able to roll over and just stretch out without the fear of falling out. I have fallen out, not clever. But if I had really very little space, I would perhaps opt for a good quality single or, better, a small double. Small doubles are ideal for one - or for a guest bed - two people for a couple of nights.
Most important is the bedding. As has been said here, you need to make sure that the fitted sheets fit the depth of the mattress, that you don't skimp on anything that is going to tuck in and for duvets really to be comfortable, should overhang. A just fitting duvet on a small single bed is a nightmare since you want to turn under the duvet through the night, not have the duvet turn with you and then get dislodged. A big bed with a bigger duvet is real luxury, but two duvets on that can be ideal since each person can choose the weight/warmth and the bedding is easier to change.
If you feel the cold, a memory foam mattress is perfect. It's possible to wake up in the same position as how you fell asleep since they are so supportive and don't encourage fidgeting. With a single bed, it's possible to wake up on the floor and, for older sleepers, easy to hurt yourself. I'd always opt for a bigger bed - even if sleeping alone.
