Your version is pretty old, but I'm sure the language editor is probably in the same place in your version, so go to the Control Panel, choose "Languages" under "Display Options" and click "Language Editor" then "Search String" for "The contents of the Signature field is too long.".
As for the other thing you're trying to accomplish; set the default "user group" to have a signature length of 0, then setup a new group and set it to what you want a "Full Member" to be and then set that group to allow users after X amount of posts (20 or so I guess)