Why am I getting "Message is too long (maximum is 65535 characters)" when I am trying to create/send a new message?
When you see the error message "Message is too long (maximum is 65535 characters)", it is likely because you are attempting to copy the contents of a Word (or similar) document into the message editor, and the content contains images.
When copied and pasted, these images get embedded in the editor as very large strings of characters. Each image can have thousands of characters.
It is alway better to copy and paste just the text, the insert the images using the image icon of the editor 
Word and similar applications often include a lot of formatting and hidden text strings that you would not want included in your messages, as they can be difficult for some email clients to interpret/display.