The plugin I am going to cover in this post, is an existing one which has been standardised; Comment Validator.
This plugin is a fairly simple one which prevents comment shorter or longer than admin defined lengths being submitted. There is also an option to prevent unregistered users using the name of a registered user when posting.
To try this comment, try leaving a comment on this post which is shorter than 10 characters.
The plugin can be downloaded via my Development site.