Our bunkbeds for grandsons are up against a wall and are not easy to make neat. One must step on ladder to make top bunk, and reach over around safety rail ( if there's one), and on bottom bunk, reaching in without banging your head is a challenge! My solution would be to make it easy on mom and safe for kids to make bed. One side should have a " drop " of 8 to 10" ( top one can be tucked while bottom one hangs loose. The rest should just cover the 48x 72" surface. A little ( 8") longer may be ok if end of bed is visible and will need tucking. So l suggest 58 x 72 to 80".