The beds of college dorm rooms and summer camps usually have twin XL mattresses. Measuring about 39×80 inches, this mattress is the standard size you will find in your campus apartment or dorm room. Some might even have a twin size mattress, but twin XL is the general norm. While buying things for your dorm or if you are getting ready for a summer camp where beds will be provided, make sure you carry twin XL sheets with you.