When I compose a new Gmail message, the text defaults to a small font size.
How do I reverse this and make it use the normal-sized font again?
I'm not sure what I've done to cause it to use the small font in the first place.
Under "Settings" on the "General" tab, you should see something like this:
This allows you to define how your default text looks: which font is used*, font size (Small, Normal, Large, Huge), and font color. (There is an additional button to remove existing formatting you may have applied.)
If that doesn't do what you need you may need to increase the font in your browser. (Ctrl+mousewheel is probably the quickest way to do that.)
* currently Sans Serif, Serif, Wide, Narrow, Comic Sans MS, Courier New, Garamond, Georgia, Tahoma, Trebuchet MS, or Verdana
Hope this helps.