Solar energy is at the forefront of renewable energy sources. Depending on the increasing energy need, the use of clean energy sources is inevitable in terms of human health and the environment. The application of solar chimney power plants (SCPPs) is one of the not very old and promising systems. SCPPs are systems that attract attention with their long life and simple working principles. SCPPs have three basic elements: collector, chimney and turbine, and optimizing these elements in terms of design and operational parameters plays a key role in the performance parameters of the system such as power output and efficiency. This study, which references the Manzanares pilot plant, aims at the collector dimensioning of the system to achieve the optimum power output and efficiency from the plant and to assess the upper thermodynamic limits of the Manzanares pilot plant. The most challenging aspect of SCPPs, in general, is their high cost, and large collector areas constitute a notable fraction of this cost. Therefore, for performance and cost optimization, collector dimensioning is of vital importance and reveals the innovative aspect of this study. For this purpose, a 3D 90° computational fluid dynamics model is created with ANSYS engineering software. RNG k-e turbulence model and discrete ordinates solar ray-tracing algorithm are applied to the model. In the study, solutions are taken in the dimensions of the pilot plant with an ambient temperature of 290 K and two different radiation intensities of 800 and 1000 W/m2. The collector radius (R_coll) is increased up to 2 times the reference size and its effect on the system is evaluated. It is found that the plant, which produces an electrical power of 46.6 kW at 800 W/m2, will generate 138.3 kW of electrical power if the R_coll is doubled. The optimum collector radius rate (X_coll) is found to be in the range of 1.00–1.50. For the greater values of X_coll than 1.50 (183 m), it is observed that the exponential increase in power output stops, and cost-related upper thermodynamic limits are achieved.