I noticed in Yosemite, Messages has two menu options under the View menu, "Make Text Smaller" and "Make Text Bigger." But unfortunately neither option does anything at all. Is this a bug? Do I have something in my settings preventing this from working?
In Yosemite Messages.app, selecting Text size: Other... brings up the Fonts picker window, which lets you select any font and size you desire:
Once you choose the font you want, it appears in the Text size selector:
Under 10.10.2 when you're typing inside a message to be sent those menu options function and increase/decrease the size of messages that appear above the text input. The text input itself doesn't actually change text size though.
In Messages > Preferences there's a slider that does the same thing.