Shoutbox

First of all, you will need StuffPlug-NG, get it at stuffplug.com and install it. (Validation key can be found on stuffplug.com, look for the link)
Then enable the feature via preferences> plugins > stuffplug ng > configure. After that just add a big display picture and you will be prompted if you want to leave it the same size. (You won't see a bigger DP, just your contact will see it)