Rule 2.6.4 from the ITTF handbook says:
From the start of service until it is struck, the ball shall be above the level of the playing surface and behind the server's end line, and it shall not be hidden from the receiver by the server or his or her doubles partner or by anything they wear or carry.
I have been told that the ball can't be out wide on either side even if it is behind the server's end line. Is that wrong? Can you serve from as far out wide as you like?