As the sizing of borehole heat exchangers (BHEs) is crucial for ground-source heat pump systems, which are becoming increasingly complex and diverse, novel sizing tools are required that can size both boreholes and connected systems. Thus, an optimization-based sizing method that runs in TRNSYS with other component models is proposed. With a focus on the feasibility of the method for typical BHEs, the sizing of irregularly placed boreholes using the well-known duct-storage (DST) model that inherently cannot describe irregular borefields is examined. Recently developed modification methods are used for the DST model. The proposed sizing method is compared with the existing ground loop heat exchanger (GLHE) sizing program. The results indicate that the proposed method has a genuine difference of approximately 3% compared with the GLHE, and the difference increases with the thermal-interference effects. A regression-based method selected to modify the DST model for describing irregular borefields exhibits acceptable sizing results (approximately 5% for test cases) despite the genuine difference. This study is the first to use the DST model for sizing BHEs under irregular borefield configurations, and the tests indicated acceptable results with an approximate difference of one borehole among a total of 30 boreholes in the test cases.