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
I know, I know. You can't post comments yet.
I don't know if there are other text size options, but according to this Meta SE Q&A, 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.
