Full size beds (a.k.a., double beds) are wider than twin size beds (a.k.a., single beds), but both are the same length. Twin beds are useful and affordable for young single people, but full beds are more likely to function adequately for couples.
Both full beds and twin beds are 75 inches (191 cm) long. They differ when it comes to width, however. At 54 inches (137 cm) wide, a full size bed is nearly as wide as a queen size bed. Twin beds are much narrower, measuring a mere 39 inches (99 cm) across.
This video provides information on all bed sizes, including twin, full, queen, and king:
Both full and twin beds have extra-long, or XL, versions, which add an extra five inches (13 cm) to the bed's length. At 80 inches (203 cm), this makes these versions of full and twin size beds as long as king and queen size beds.
In the U.S., all mattresses and beds manufactured and sold by IKEA are a half-inch to inch (1.3 cm to 2.5 cm) shorter than is standard. Buying a twin or full size mattress, bed, or box-spring from IKEA more or less locks the buyer into buying IKEA's bed products only.
Pros and Cons
Twin beds are ideal for children and are often found in bunk beds. Twin size beds are also good for teenagers, college or university students, and those who want a small bed for a small room. These beds are affordable and very easy to move, making them somewhat more flexible than other bedding options, which can be expensive and burdensome. However, few adults will want a twin size bed for their main, personal bed. A twin bed is only meant for one person and will be impractical and uncomfortable for most adults to share with a partner.
Though more expensive, a full size bed is more likely to "grow" with an individual and can even function adequately for some couples, allowing each person 27 inches (68.6 cm) of personal space. In the long run, most couples will likely want a more spacious queen, king, or California king size bed, though.
Comparable full size and twin size mattresses and bed frames are often similarly priced, with usually only around $50 separating them. Other cost variations result from material differences (e.g., whether a mattress has a Eurotop or pillowtop or whether a bed frame is made from hardwood or softwood). One noticeable difference between the two bed sizes is that full size sheets often cost twice as much as twin size sheets, but this is a relatively minor cost overall.
In at least one (older) survey, twin size beds were found to be the second most popular bed size in the U.S., surpassed only by queen size beds. Full beds were moderately less popular at the time of the survey.