Futon mattresses come in twin, full or queen. They are made to be close to the same size as a conventional mattress. Because they are some what "unstructured" they are not going to be exactly they same size - close but not exact. Measure the futon mattress & use standard sheets & a mattress cover - just as you would a mattress. As stated previously, you will have to tuck some of the excess - today's sheets are made for the deep, pillowtop mattresses.