Minimum characters per visitor message?

[PinkMilk]

Updated after reading OP again

I'm not sure you can change one without the other, my quick fix suggestion would be to add a guest condition to editor_toolbar_off template and add the pattern and required attributes to the textarea:

Find:

tabindex="1">$newpost[message]

Before add:

<if condition="$show['guest']">pattern=".{10,}"   required </if>

Then change 10 to whatever amount you wish min to be.

This is just a quick fix, I'm sure someone else would come up with a better solution.