Loft Bed Dimensions are generally twin loft 39 inches x 75 inches, twin XL loft 39" x 80", rarely, Queen loft 60" x 80" and more rare yet Full XL loft 54 inches by 80 inches. These are fairly standard loft bed measurements. Loft Mattress depths can vary from just a few inches to 8" or more, very dependent upon the height of the safety rails. Many times, actual loft mattress dimensions will vary by one or two inches, usually smaller.