Treating a CMC service just like the telephone, paper mail, or any other medium will lead to misunderstandings and mistakes. Emulate experienced users. It's not just a machine. All that is in front of you may be a piece of hardware, but there are people on the other end of CMC services, and there are people responsible for maintaining and developing resource sharing services. Using many words is more likely to cause misunderstandings than using a few well-chosen words.
People are also less likely to read long messages.
More than a page or two is probably too much. When responding to a message and including part of it for context, include as little as possible while maintaining clarity and precision. Label your message. Choose a title that fits the subject and stick to it. If you need to bring in another subject, consider posting an additional message.