If you're talking about per-text size changes, there's some basic things you can do.

For large text, you can use a `#` before the text.

`#Hello_World` goes to 
#Hello_World

It needs to be on its own line.

The small text you see in Oli's answer was produced by putting `<sup></sup>` or `<sub></sub>` around the text.

    <sup>I know, I know. You **can't** post comments yet.</sup>

to  

<sup>I know, I know. You **can't** post comments yet.</sup>

I don't know if there are other text size options, but according to [this Meta SE Q&A](http://meta.stackexchange.com/questions/170480/is-it-possible-to-change-the-text-size-in-the-code-markdown), there don't seem to be any more options.

Edit:

Thanks to Videonauth, I can now say that using some basic HTML tags will work, such as `<h1></h1>`, `h2`, etc. I think these will display differently on different browsers, depending on whether or not AU/SE has them customized in the CSS.