Centre line

In double serving how much of the ball has to be over the line? Is it only the centre part of ball or a fraction of it? Unlike tennis where the ball  compresses can the the serve be in if any part of it is over the line?

According to the rules for a doubles serve to be considered legal, the ball must touch the right half portion of the table for both the server and the receiver. When we talk about the right half, this includes the center line that divides the table into two equal halves. For your specific question about the ball's position relative to the center line during a serve in doubles, the rule states that at least some part of the ball must be over the right half of the table at the point of contact. There isn't a specified requirement for how much of the ball needs to be over the line. So, if any part of the ball is over or even just touching the center line at the moment of service contact, the serve is considered legal.