Twin mattresses are 39 by 75 inches. These mattresses can fit through small spaces such as small stairwells and around sharp corners. The mattress is only meant for one person. If you need a bed for two people, there are queen- and king-size mattresses that will fit in a small stairway.
Queen-size mattresses are 60 by 80 inches. If you have a small staircase, the queen mattress may not fit. The split-queen mattress is 30 by 80 inches. The width is smaller than the twin mattress, but 6 inches longer. This mattress would fit through a small staircase that can fit a twin mattress.
King-size mattresses are 76 by 80 inches. The size of the mattress makes it harder to fit through tight spaces including stairs with low ceilings. The split king is 38 by 80 inches, which is only 1 inch smaller in width than the twin mattress. This mattress would fit in a small stairway.
The bedsheets for split-king and split-queen beds are the same bedsheets used for one-piece mattresses. The mattresses fit together tightly on the bed frame, so you can use regular sheet sets. There are sheet sets with two separate fitted sheets for each mattress and one flat sheet, but these are more expensive than those with one fitted sheet, a flat sheet and two pillow cases.