Setting Emacs Font Sizes
Changing the font for a single buffer
To change the size of the font for a buffer, and all the buffers of that kind, type <shift><command>T to get a menu for fonts, and click on whatever size font you want in the right hand column. I use 18.
A longer way to do the same thing is to click in the menu bar: Options > Appearance > Font for <...> mode. This also brings up the menu for selecting fonts and their sizes. It will change the font for whatever type of buffer you're looking at. The abbreviation <...> will be "text" if you're looking at a text buffer in emacs at the time. Otherwise, it will be something else.
Saving the changes
I changed the size of the font for lots of different types of buffers: apropos, emacs-lisp, shell, etc. For instance, to change the size of the font for the Apropos buffer, do M-x apropos, then give any topic, e.g., search. Now you should be looking at an apropos buffer, and you can change the size of the font as described above.
To save these so they will be the defaults for future use, do > Options > Save Options
That saves the font specifications in: ~/Library/Preferences/Aquamacs Emacs/customizations.el