Wanting to start on a shortboard?

I know it's not ideal for beginners, but I'm 5'5 125 lbs and 16 years old, and want to learn how to ride a shortboard. I like riding longboards too, but I want to try shortboards. I can switch on and off between the two if a shortboard gets too frustrating, as many say. If I wanted to try a shortboard for the heck of it, could anyone recommend dimensions? I know it's certainly not ideal for beginners to buy their own boards especially since they don't know what the want, but what do you guys think about the CI New Flyer (5'5 - 18 3/8 - 2 3/16 - 22.8L)? What would be the best dimensions for waves in the San Diego/Encinitas area? More specifically, more beginner-like areas such as Beacon's or Carlsbad (surfed there before). Surfline averages the waves at 2-3+ ft and says knee high to a few feet overhead. I usually go in when the tide gets too high.