A full size mattress is 54 inches wide by 75 inches long. The depth of the mattress can vary from manufacturer to manufacturer and can be an important consideration when purchasing sheets. Extra-deep mattresses require sheets constructed to match, or queen-size sheets can be used and tucked to fit.
After the introduction of the twin XL mattress size, some manufactures started producing extra-long full-size mattresses. These mattresses add an extra 5 inches in length to accommodate taller sleepers, matching the length of a queen- or king-size bed. Regular full-size sheets do not fit a full XL mattress due to this extra length.