First, the beach slopes very gently out to sea, so you can stand up right where you surf. You can see that in this photo. The instructor is standing and is ready to give the student a push to help her catch a wave.
Second, you can regulate the size of the waves you want to surf. To the north end of the beach, there are bigger waves that roll in from the open ocean into Tamarindo beach. On the southern end of the beach, the waves are smaller because there is an off-shore island and a volcanic reef under the water that partially shelters the southern end of Tamarindo Bay from the full force of the ocean waves. (The partial protection from the island and reef is what enables boats to be moored in the waters of the southen part of Tamarindo Bay, as you can see in this photo.)
Third, Tamarindo has lots of surf shops where visitors can rent surfboards and get surfing lessons. I often see small groups of tourists getting surfing lessons and progressing from riding the waves while lying down, to kneeling on hands and feet, to standing up.