Many online resources offer mattress dimensions, and Consumer Reports is one such resource. As of 2015, its website offers standard mattress dimensions for king, California king, queen, full and twin size mattresses.
Better Sleep Council also lists dimensions for mattresses, including a twin extra-long (XL) mattress. Sleepy's, a mattress retailer, offers a mattress size guide that also includes a crib size mattress. Some of the other online retailers that offer mattress sizing guides are Serta, BackToBed and BeddingExperts.
Standard size mattress dimensions are 76 inches wide by 80 inches long for a king; 72 inches wide by 84 inches long for a California king; 60 inches wide by 80 inches long for a queen; 53 inches wide by 74 inches long for a full/double; and 38 inches wide by 75 inches long for a twin or single, as listed by Consumer Reports. A crib size mattress measures 27 inches wide by 52 inches long, and a twin XL is 39 inches wide by 80 inches long, according to Sleepy's.