Whether your full-size twin bed frame metal is an antique or a brand new, it may be a little too small. Especially, if more than one person sleeps in it every night. If you want more space, but want to keep your bed, you can install a special converter kit that will allow you to use a double mattress in your full-size headboard and footboard. A marriage mattress is 6 inches wide and 4 inches longer than a full-size mattress, which gives a little extra space.
Look at your headboard and feet to see if your bed rails are joined with screws or with hooks. Buy a converter kit that connects in the same way. Take your full-size bed apart. Remove the mattress, box spring, and rails. Attach the rails of the conversion units to your headboard and foot. If your rails have hooks, slide gently into the slots in the headboard and foot until they slide into place and you feel resilient.
Move out the middle support rods and secure them together with the included bracket. Place two side brackets on each side of the rail, using the included hardware and pre-drilled holes in the rails. Place double the box spring on the converter rails. The box spring will extend beyond the width of the head and foot a few inches on each side. Squeeze the side brackets so that the box spring will hug tightly. Locking the side supports in place using wing nuts or other hardware provided. Place the mattress on top of the spring box.