If you are communicating with another professional or a firm and the level of communication is professional and legal, always keep your signatures long. A standard space for long signatures covers about four to six lines of the document or page. Remember signature of a person can be taken as an evidence in a legal battle, so better try to put in detailed/complete signatures if you are communicating at professional level. This format normally encompasses the end of your message/letter with closing salutation i.e. Kind regards/Yours sincerely
If you are signing a document i.e. a Memorandum of Understanding (MoU) you will have to put in your signatures on all pages of the document. In such cases you are required to put in your signatures and date that you are signing the document on. No salutation or other details are required.
You can also create short signatures, especially if you are communicating with colleagues or with an office in another country or city on a regular basis. In this formation your signatures should go beyond two or three lines. Since you are in communication with the recipient of your email frequently, there is no need for a lengthy signature. In this format for example you can
Your Phone Number
Generally, companies have a set format for all employees, and IT department helps create common standard signatures, some times they themselves create email account of the employees with signatures inscribed in them. So in that case an accepted format has to be followed and you can't have much say about it.
Signatures for Internal Communication
Again, you can further shorten your signature to a couple of lines. It should include a salutatory message i.e. take care, and then your name. This is normally used for communication within an office and generally for an informal messages. Also, if you are communicating with your boss or worker on a regular basis it is the most commonly accepted format.