I know that it was possible to boost the Personal Message length on previous versions of Messenger by doing a resource (or hex) edit
(All the patches stole this function off me! Those asses 
) but I'm not sure if it's still possible. There was a max length checked by the server itself which was significantly larger than that allowed by the client.
As for nicknames I believe in the past you could type
/nick <name> and it would allow you to have larger nicknames, I might be imagining this (as I'm not a person who uses excessively long nicknames) and I'm not sure if this is still possible or not.
EDIT: I just checked some patches' features and they dont seem to have the boost PSM feature any more so I assume it may no longer be possible (or at least as simple as it used to be without manipulating protocol)...
/ 
