The different sizes are for different applications. The dreadnaught was designed in the early 30's for the purpose of volume. They wanted to compete with banjos and fiddles. That's also why the dread is the guitar of choice for bluegrassers. The OM's 000's, 00's and such are, some say, sweeter, not as "boomy" in the low end and more balanced which is desireable for fingerpicking.
The whole thing is subjective, though. I've found when looking for a guitar, play what you normally do, play as many guitars as you can find and the "one" will find you. Just be aware that that one might be a more expensive model than you planned on. I like dreads, personally, but I prefer to play standing up so it's much easier that way.
I also own a Martin "M" style guitar which is also called a 0000. It is the size of a jumbo but the depth of an OM, like the Mayer. It feels like a small guitar but can deliver the tone and sound similar to a dread.
There's just so many kinds you can try new guitars for years and keep finding different things you like and dislike. And that, my friend, is a great way to have fun.