The difference between a twin mattress and an extra-long twin mattress is the extra-long mattress is 5 inches longer, states Sleep Experts. A twin mattress is 75 inches long by 39 inches wide, while an extra-long twin mattress is 80 inches long by 39 inches wide, notes The Bedding Experts.
The extra-long twin mattress is the same length as a queen-size or king-size mattress, according to Sleep Experts. A standard full-size is also 75 inches long though it also comes in a longer, 80-inch version, notes Sleep Experts. Both twin sizes are meant to accommodate one adult, while the full, queen and king can all accommodate two adults, according to The Bedding Experts.