The built-in Mac OS X applications—like Mail, Preview, and TextEdit—all use the same Fonts Window to help you change the style of your text. iWork uses that system, too, as do a number of third-party programs (like TextWrangler). So here are some things you should know about how it works.
While you’re creating a document, hit Command-T at any time to invoke that Fonts Window.
In Mail, for example, if you select some text and click a font from that window, it’ll change the selected text; if you have no text selected, the font you choose will apply to whatever you type after your cursor.
The first great thing about the Fonts Window, though, is being able to see what the fonts look like as you go through the list. To do that, click and drag down the little circle at the top-center of the window to reveal the font preview…
…or click on the window’s gear icon and choose Show Preview.
That way, you can know what any particular font looks like right from there, before you go about applying changes willy-nilly.
Also under that gear menu is Edit Sizes, which’ll let you add in font sizes that don’t appear by default.
To do that, just enter the size you want in the New Size box and click the plus button.
I know, I know, that’s backward from pretty much everywhere else in OS X. I definitely tried to click the plus button first a few times.
Within Edit Sizes, you can also set the range for the slider that appears on the right side of the Fonts Window.
Other options available under the previous gear menu include adding the selected font to your favorites list and viewing special typographical features for any particular font.
(I think you’re probably a way fancier font person than I am if you use these advanced typography choices.)
Note that changes you make to the Font Window are application dependent, so what you configure in Mail doesn’t apply in Pages. Which is kind of a buzzkill, but I suppose you might need your font-size slider to be a different range in various programs. I’m gonna give Apple the benefit of the doubt on that.