The full-size or double bed is 54 inches-by-75 inches. That means that each person has only 27 inches of sleeping room, which isn't a lot of space, especially for people with larger statures. The length is good for people who are less than 6 feet tall, depending on where you place your pillow, which is 20 inches wide.
Because full size beds are smaller than queen or king size beds, you can fit the mattress and box spring into a smaller room. If you have an upstairs bedroom, the mattress will maneuver up the stairs easier than a larger mattress, especially if you have a low ceiling or sharp turns in the stairs.
Bedding for the full size bed is cheaper than queen or king size bed sheets. Full size flat sheets are 80 inches-by-94 inches. The fitted sheets are 54 inches-by-75 inches and the standard pillow cases are 20 inches-by-30 inches. Comforters for a full size bed are 80 inches-by-90 inches, or you can use a full/queen comforter, which is 88 inches square.
Queen size beds are 60-inches-by-80 inches, which gives each person 30 inches of sleeping space. Queen beds are 5 inches longer than full size beds, to accommodate taller people. The bedding will cost a little more, but if you have a full/queen comforter, it'll fit the queen size bed.