Messenger Plus! has that feature built in
You can find it in the options panel. But you can go upto only 1100 characters.
If you want more then download Stuffplug from
the downloads page on stuffplug.com (download StuffPlug-NG 2.2.397 BETA which is the latest version. It works perfectly with 7.5 dont worry
)
There is an option in there to boost your characters to 5000, but for that you first have to switch off the plus! option to boost it to 1100
/ 
