I've enabled CAPTCHA for people to leave comments, and it's working okay. Only problem is the CAPTCHA display (the letters and numbers you're supposed to retype) is very small. I'm trying to figure out how to make it bigger, but any pointers/suggestions would be appreciated.

I second that. I'm just getting started with ZenPhoto and really like it, but that is something I noticed right away. You almost need a microscope to read the symbols.