Hey it looks nice indeed.
I don't think you realy needed this:
} else {
$this->size = 80;
}
in the setSize function, as the gravatar.com will return the "80" size anyway if the var is larger than 512.

Anyway... here are some challenges:
1. can you also think of a php4 compatible class (if possible... with cache option)?
2. can you think of a "gravatar sign-up" class, so the user sign-up directly using a form on your site and not being sent to gravatar.com? (http://txfx.net/code/wordpress/gravatar-signup/ might be a good start, but that plugin is made for WordPress and not Generic )