Othrothopedic surgeons say 16 1/2 for the Slider. The Slider is NOT a better pitch than a curveball. It is a different pitch from a curveball.
To throw a curve ball-you must first make sure you are getting the proper spin on the ball. If you are throwing over the top it should be spinning from 12 to 6 ( have over spin). Point your thumb at your ear and make a “C” with your index and third finger. The palm of your hand should be facing your ear. Now extend your arm in front of you. Keep you wrist flexible and neutral -not hooked. When you accelerate the baseball will tumble out of this little pocket (the “C”) you have made. It should leave over the first joint of your index finger- not the tip. One you have done this and have overspin on the ball-think a little about arm motion. Think about a low/flat arch (in your motion) rather than powering straight though with a fastball or a slider. Hope this helps,Ian.