If you are a shorter player, chances are you have been told over the course of your lifetime that you need to give up the idea of having a big serve and settle for getting a high first serve percentage. There is nothing wrong with this line of thinking if you have maxed out all your options (practiced for hours and hours) coupled with intense hunger (relentless energy and willingness to learn), which means you have given it your best effort and your best option is to just start the point with a safe slice serve.
However, if you are hungry enough, young enough (adults, it's never too late!), and lucky enough to have a equally motivated coach who knows what they are talking about (proof is in the pudding, they have a great serve themselves) - shorter players can have great serves. To be honest, the serve is not as complicated as people (usually teaching professionals) make it out to be. Here are some simple tips:
1) Continental grip
2) From beginning to end of stroke, your grip on the handle should be a 1 or 2 out of 5, 5 being tight (Kind of like holding a bird in your hand, don't want to kill it nor do you want it to fly away)
3) Swing as fast as you can (a loose arm and wrist will creat a wrist-snap)
4) Do not move your feet (simpler-the-better to start, less moving parts that could breakdown)
Without fast racquet headspeed, your serve will never be great. It's as simple as that. The serve is a trust shot, full commitment. Once you get the racquet headspeed, then developing a legit serve can happen. With the racquets of today, shorter players can have monster serves, monster kicks, and monster sliding serves.
Here are some creative ways to improving your pop on the serve:
1. In the first drill, try to stay close the back fence on one side of the court ("S" for "server" get it?) and aim at first directly into the opposite back fence. Try to hit 10 in a row without any arc whatsoever...that is, straight as a bullet; fence to fence. After you hit ten, aim the next set inside of the opposite baseline (#2). Thereafter, 10 inside of the service line. Again, try to focus on power, not on hitting (reaching your target) with arc. The purpose of this drill is to isloate the fast-twitch muscles in your shoulder and core.
2. In the second drill, start out at the fence again and hit ten serves inside the service box. After your first set, move in 2 feet and hit another ten. Then move in 2 feet again. Work on hitting "up and through" the ball. As you get closer, that power that you're developing will be channeled into a proper technique.
3. The "machine gun" drill is also a good way to loosen up and isolate the shoulders, arm and core. Grab 4 or 5 balls in your left hand and serve those balls (from service line) in rapid succession: toss and hit; toss and hit. Hence, the name of the drill. Feet should remain firmly planted on the ground.
4. Reader "tweener" has suggested that to work on pronation, the server should practice serving from the knees. This is also a good way to isolate the shoulders and work on developing the forearm strength. Thank you, tweener, for your advice. This is the type of input that we're looking for on this site.
5. Lastly, it's important to recall our previous tip regarding power. If you want to improve your serve speed, you must practice hitting fast serves. If you want to run a fast sprint, don't train for a marathon. Speed of the arm and power generated by the body need to be integrated into the motion. Assuming that your serve components are there, start cranking the serve. Since serving "hard" is draining, appropriate rest should be taken until your body builds a tolerance to this type of workout. Bear in mind that a lot of players' career have been stunted by the mentality that "I'm short and therefore I'll never be a good server". This is like saying "I'm tall and therefore I don't need to practice my serve". What's one thing have to do with another?! Sure, certain players might not hit 150mph. But it may be possible for them to hit 120s or 130s. However, they're never going to get there if they settle for 90s in practice. Boundaries of your physical abilities must be continually pushed - little by little.
If you have the tennis bug and you are willing to put in the work, don't settle for a serve that is slower than falling snow. The time will be well spent and I've never heard of anyone who doesn't like to win.