It's definitely an advantage to be tall in tennis because then your serve has more power(the tallest of the pros usually have the biggest serves), you can cover more at the net(high lobs and volleys), and you have more reach. Sometimes though if you're very tall you're more ackward at footwork and moving around. You don't have to be very tall to play well though. I'm 5'6 and I've played against short girls who played very well. You don't see many tennis professionals around my height though(Justine Henin-Hardenne being an exception).