There are other sizes but they are very close to one of the above and much less common. The ones above are what you'll usually see. So which one is right for you?
The dreadnaught is by far the most common body size in circulation today and has been for at least the past 40 years. This is because dreads are usually fairly loud compared to smaller guitars, which is something that many people seem to want when they progress beyond the absolute beginner stage. Some people even refer to their dreads as "banjo killers," a reference to the ongoing battles of banjo vs guitar in a bluegrass band. Just as a sidebar - the term dreadnaught was first used by Martin because way back a hundred years ago, U.S. battleships called Dreadnaughts where some of the biggest, mightiest warships and to many people the name implied something big and powerful. Interestingly, when Martin first produced the dreadnaught or "D" series guitars they were listed as "bass guitars" because compared to the much more common smaller bodied instruments of the first part of the 20th Century they sounded quite low and boomy. About the same time the Gibson company, Martin's primary competitor began producing large bodied instruments that they called Jumbos - a reference to the famous P.T. Barnum circus elephant. Both of the terms (dread and jumbo) stuck and today Martin "D" size and Gibson "J" size are the largest standard run guitars that each company produces and remain hugely popular.
But the reality is that these big bodied guitars are just too big for some people, especially most women and youngsters. You will see some women with the bigger guitars - Sheryl Crow and Emmy Lou Harris prefer Gibson J-200s and Joni Mitchell is almost always seen with Martin dreads - but they are by far the exception. Still, dreads and jumbos probably make up 90% of the guitars imported from China, Korea, Japan and elsewhere.
Things may be changing though. Taylor makes a traditional dread size guitar but far more popular are their slightly smaller and more rounded 7-, 8-, and 9-series instruments. They make what is essentially a jumbo that they call a Grand Concert (GC) - which is a very nice guitar by the way, in spite of my aformentioned dislike of Taylors - but it is not all that popular. Martin and many of the boutique makers are putting much R&D and new model launches with 000, OM and smaller instruments. The reason is simple. They are just easier to hold and reach around and play. True, they do not have the volume of the jumbos, in most cases, but what they lack in boomy volume they more than make up for in crisp, clear treble and an overall much more even and blended sound. This is particularly attractive to players who are soloists who play finger-style.
Smaller still are the 00 and finally what have become known as "parlor guitars." The 00 was hugely popular during the so-called folk revival of the 1960s and before that many people bought the Martin 00 and Gibson L size because they were reasonably priced compared to the dreads and jumbos and for most beginner and intermediate players, just easier to play. The smaller still parlor or O-size guitars are for most people something of a curiosity, that is, a nice alternative to the larger guitar they probably own, fun to use as a "couch guitar" and very comfortable but generally lacking in much volume and certainly not appropriate for ensemble playing. A few companies are making what they call travel guitars, which are a 3/4 size instrument that in most cases can fit in the overhead on an airplane when carried in a soft gig bag. I owned one for a while, a Martin LXME and it was surprisingly nice sounding guitar. These also are a GREAT type of guitar for children to learn on because they are small, usually made at least partially of man-made materials meant to take some abuse, and sound pretty darned good. They are NOT toys, even if they appear
So which one is right for you? If you're a fairly large person and can comfortably reach over and around the body of a jumbo or dread, that's great - you have many, many more choices when you go shopping for a new guitar. If you're not so large a person, try to look past the volume of those jumbos and think in terms of comfort. Go with a 000, Concert, or OM size. You'll be able to strum loudly enough to not get drowned out when jamming with your friends but the clarity of the treble end of your guitar will most likely make those dreads sound pretty muddy by comparison. As they say, YMMV. Just try a bunch of guitars before you form a definite opinion, but always be ready for a GAS attack (Guitar Acquisition Syndrome!) and don't be surprised if that "perfect" guitar turns out to be ... not quite! Just part of the adventure of learning the world of guitars.