Commonly asked and even more commonly confused are the Full size mattress and the double size mattress.Both have been around for years but are the full and double size mattress the exact same size, or is one bigger than the other? Buying a too-small mattress is one of the biggest mistakes new bed-buyers make. Couples who’ve been sleeping on a “double” (also known as a full-size mattress) since they got together may think they have enough room – until they learn that each person has only as much sleeping space as a baby’s crib! I purchased this mattress in September 2017. In July 2018 (less than a year after owning this mattress) I noticed there was mold on the underside of it. Loft Bed Plans. Instructions for how to build a Loft Bed twin-sized standard height. This Loft Bed Plan measures 65 inches in height, by 42″ wide, by 81″ long with about 48 inches of clearance beneath. December 2018 Update:. The Sleep Advisor team is excited to announce our updated mattress picks for 2018! .