High-density circulating fluidized bed (HDCFB) offers high gas-solid contact efficiency and shows great application prospects in the petrochemical industry. It is important to study the scale-up of HDCFB for its successful industrial application. Therefore, the gas-solid hydrodynamics of the full-loop HDCFB with different riser heights and riser diameters was systematically investigated by coupling a Euler-Euler model, an energy minimization multi-scale (EMMS) drag model, and a modified solid pressure model. Results demonstrated that the solids circulation rate of the HDCFB system decreases with increasing riser height. When the riser height is 10 m, it is difficult to achieve full development of gas-solid flow. When the riser height increases to 18 m, full development is achieved at a normalized height (h/H) of 0.35. At high solids circulation rates, high riser heights are required to achieve sufficient gas-solid flow development, and adequate storage height is needed to provide the driving force for high-density operation of the circulating fluidized bed. As the riser diameter increases, the pressure drop in each unit of the HDCFB system changes linearly, with the pressure drop in the riser decreasing and the pressure drop in the cyclone separator increasing. In large-diameter risers, the radial distribution of particle concentration and particle velocity inside the riser becomes less uniform, and larger particle clusters can form near the wall with severe particle back-mixing. When the riser diameter increases from 80 to 150 mm, the normalized height of the fully developed gas-solid region increases from 0.15 to 0.42 (h/H), indicating that particles and clusters in large-diameter risers need a longer acceleration zone to develop a stable flow pattern. These findings can provide useful information for the design and scale-up of HDCFB systems.