If you are sending a message that more than 10.000 character you will receive a warning message that says
If you do not like limits you can just simple go
Admin --> Options --> Messages

And find Maximum Message Lenght as below:

For my case I increased 10 times. From 10.000 to 100.000 character.
My suggestion is not to increase more that 100.000. It should be quite enough normaly.
Please enter a message with no more than 10000 characters
It's just a precaution to prevent spam posts by Xenfro.If you do not like limits you can just simple go
Admin --> Options --> Messages

And find Maximum Message Lenght as below:

For my case I increased 10 times. From 10.000 to 100.000 character.
My suggestion is not to increase more that 100.000. It should be quite enough normaly.