Whatever you do, keep it short.
On the low end 40 characters suffice. For some 160 is a minimum. Every day people send text messages from phone to phone, messages that are supposed to contain the most important of information. They write where they are, how they're doing, what they're doing. They get in touch with business partners, family and friends from around the word. For many people the SMS, or Short Message Service, has become an important communications medium. It's therefore no surprise that its abbreviation, SMS, has shape-shifted into a verb: simsen. "Just write me an SMS!" was far too cumbersome.
But why is it called simsen rather than smsen? Because simsen can be pronounced much more easily than smsen. This word of the week, by the way, can even be explained in the length of a standard text message: "Dear learners of German. Simsen is slang for sending a text message."