The plant pictured is the one you will receive. It is 2 in wide x 2.25 in tall. During hot weather you may need to water the plants more frequently so long as the plant is actively growing. From March onwards the plant will begin to grow and watering should be increased gradually until late May when the plant should be in full growth.
My research shows that this is probably a Parodia leninghausii or Yellow Tower cactus. I am not a cactus expert so there is a chance that I have mislabeled the plant. You will receive the actual plant pictured.