This is a short guide to help answer common questions regarding the differences between mattress sizes. It lists the most common sizes, common dimensions, and comments regarding each. Please note that differences in manufacturing may vary the sizes by an inch or so.

Twin (39"x75")

The Twin mattress was once called the Single. It's suitable for children, students, etc. The greatest limiting factor in this size mattress is the length. 5'10" is about the maximum height that can sleep comfortably on a twin. Anyone taller runs a significant risk of having their feet hang off the end.

Twin XL (39"x80")

The Twin XL mattress is similar to a Twin but is the same length as a Queen or King. This size is a great choice for college students, adults who need a twin width but are too tall for a twin length, or even just tall children. Expect to pay about $100-$200 more than the Twin price for the extra 5 inches. Two Twin XLs pushed together are the same size as a King and are sometimes refferred to as a "Dual King".

Full (54"x75")

The Full was once called the Regular or the Matrimony bed (because you got it when you got married). It's the same length as a Twin and therefore has the same restrictions on height. At 54" wide, however, it's 15" wider than a twin. Fulls are great choices for children or rooms where you absolutely MUST have that extra 6" of free floor space. College dorms often use Fulls. Fulls are generally either the same price as a Queen or very close.

Queen (60"x80")

The Queen was pioneered by Simmons sometime in the middle of the last century. It continues to be the most popular size though every year Kings gain on them a little bit. Queen is the first size that really requires a center support in your frame. A center support is something in the middle of your bed that physically touches the floor. Good frames have more than one. Having slats that go across is NOT enough for a Queen. This lack of proper center support is a large contributer to sagging mattresses in this size.

King (76"x80")

The King was also pioneered by Simmons sometime in the middle of the last century. It's the same length as a Queen but gives you another 16" of width. Kings require at least two center supports in your frame. A center support is something in the middle of your bed that physically touches the floor. Kings tend to have less problems with motion transfer or "Partner dips" (A partner dip is when your partner is significantly bigger than you and they create a valley in the bed that you roll into). Kings use two Twin XL boxes instead of one giant King box. This is because a single King box would have a hard time fitting anywhere. The disadvantage of this is the creation of something called a "King Bump".

A King Bump is a rise or firmer area in the center of the bed caused by the firmer edges of the boxes meeting underneath that part of the mattress. Many people interpret this as "dips" to either side of the bump. A good way to avoid this are to have a bed that doesn't use boxes such as a platform bed. Another method I've heard of consumers using but have not tried myself involves putting a piece of strong plywood between the boxes and the mattress to even out the distribution of weight. There are some mattresses that are more resilient to the King Bump than others. Springless designs such as Latex or Memory Foam tend not to feel it as much. The energy absorbing characteristics of a Tempurpedic give it a particular advantage with this problem.

California King (72"x84")

A common misconception is that a California King is bigger than a normal King. This is untrue. A California King exchanges 4" of width for 4" of length. It's designed for people over about 6'3". California Kings are commonly the same price as regular Kings. Sheets and frames are generally not that much harder to find. Many old waterbed frames are California King. Because it uses two boxes, the California King suffers from the same King Bump issue as a regular King.

Less Common and Discontinued Sizes

These sizes are mostly discontinued by the major manufacturers. You may still find a few in outlets or be able to have custom bed manufacturers make them for you.

Full XL (54"x80")

Olympic Queen (66"x80")

California Queen (60"x84")

Feel free to post any questions regarding sizes you may have.

-Alex

Hey, Alexander!
I just wanted to tell you that I appreciate your being on the site with your knowledge of the beds out there and the bedding industry in general.  The dissatisfaction you hear from us is frustration from having wasted money time and again, knowing that there are beds out there that feel good, but unable to sort through the bs to find one.  Other than you, there is only one other bed-person online to ask questions of, and he's quite biased toward Sealy and Stearns & Foster.  It is supremely helpful to many of us to be able to figure out what is in a certain bed we've tried that worked great, and to be able to re-create the feelign in our own bed.  Many of us, especially those who have performed mattress surgery, myself included, have lost faith that any of the big guys offer any product that is worth a darn. 
Thanks,
Kait