The length of a message might not seem important if you don't get much mail, but for people who receive a lot of Email they may not have time to read a lot of long messages. This way people know that it is a long message and can decide whether they have time to read it immediately. Always use a subject line and try and make it as relevant as possible to your message. This is the first part of the message anyone sees when they open their e-mail program and many people choose whether or not to read the message on the basis of the subject line. A whole screen of text is not easy to read either, so try and break up your message into short paragraphs with breaks in between.
White space makes text easier to read.
For much the same reasons don't use all capitals in a message. In the culture of cyberspace, messages written in all capitals are seen as shouting. This is because a message composed entirely of capital letters is uncomfortable to read. There are other ways of creating emphasis. Otherwise when your message is received it might break in strange places which is very irritating to read.