umm they can be 4.0 - 6.0 feet
IMPROVED ANSWER:
Tennis players vary in height. Tennis players are usually tall, especially the men. The average height for a women is around 5'9 and for men around 5'11. However the men reach outstanding heights, eg/ Ivo Karlovic from Croatia is 196cm tall the tallest male tennis player. Also, rising star Bernard Tomic of Australia who has a Croatian background (Y) is only 17 and is 194cm tall. If you'd like to find out the height of some tennis players just go on www.wikipedia.com/ and type in the tennis player you are interested in.
Thanks, i hope this is a better answer.