Maximal Characters Per Message does not seem to be working for me. It is currently capping at 100 even though the config says different. I have reloaded the plugin. Other plugins I have are BetterChat, BetterChat Ignore, BetterChat Mute & CapsNoCaps. Any help is greatly appreciated.

You might find that 100 characters might be the ceiling imposed by the game itself?

Thats what I was thinking. Couldn't see a parameter for it anywhere though so was hoping someone might know.

As ElDubyaOZ said, you might be hitting a client side limit which wouldn't be controllable by Better Chat.
Do you find yourself just being unable to continue typing, or does it simply cut your message off after being sent?

Unable to continue typing so you could be right that it is clientside.

Yes, that would be a client side limitation.