A spherical tokamak (ST) with an internal transport barrier (ITB) has the potential to be an ideal fusion power reactor because it not only offers high β performance and blanket replaceability, but also enables self-sustaining operation through a high bootstrap (BS) current. This paper investigates the downsizing of an ST using high-temperature superconductors (HTSs) and ITB. Plasma parameters are evaluated using a set of plasma burning equations, including the BS current equation and time-dependent 0D energy and particle balance equations, assuming that the energy confinement time is given by HH multiplied by the IPB98y2 scaling law. In reactors using Nb3Sn magnets, the minimum achievable size is determined by the β limit due to the lower maximum magnetic field. In contrast, HTS reactors can operate at higher magnetic fields, providing a greater margin for β, so the minimum size is determined by confinement conditions such as the HH factor, impurity concentration, and α power density. As a case study, we present a conceptual design for an HTS-based ST reactor named JUST-3 (R= 2.9 m, A = 1.7, Bt = 2.6 T, IBS = 17.6 MA, Pf = 1 GW).