How can I change the default font size for composing in Sparrow?
I figured this out:  

I had a problem that my font got changed size 11, and changing back to 13 didn't fix the default - the next email would go back to 11

The solution is this: Type some text and highlight it.  But instead of changing the font in the pulldown above the editing window (which won't change the default), so this instead:

With the text selected, in the top menu select Format > Font > Bigger or Smaller, till it get the size you want.  (You can also do this by hitting Command + or Command -.)

Oddly, that fixes the default, while using the normal size toggle doesn't.