If you’re unsure how to choose a mattress, here are some important things to know about the differences between a twin XL and a split king. Split kings are generally larger than twin XL mattresses, measuring 38 inches by 80 inches. These beds are popular in guest rooms, children’s rooms, and college dorms. You may also want to consider purchasing a twin XL mattress for your family. If you have a small apartment, this size mattress may be the best choice for you.