In the U.S., a single and a twin are the same size at 39 inches wide and 75 inches long. An extra long twin is the same width, but contains 5 inches in length at 80 inches long. A twin takes up the least amount of space, but only has room for one person. A twin bed is an ideal choice for younger children or smaller, single adults. The extra long twin is commonly used in college dorms where the compact size with added length is good for taller students.