Concrete-encased steel (CES) columns are widely used in high-rise buildings, and their load-bearing mechanisms under fire conditions are of critical importance to structural safety. With the increasing application of high-strength steel and high-strength concrete, the ambient-temperature load-carrying capacity of CES columns can be significantly enhanced, while material degradation at elevated temperatures becomes more pronounced[1]. This leads to a mismatch between strength enhancement and fire resistance, calling for a re-evaluation of existing fire design approaches[2]. Under this context, geometric characteristics, particularly the overall cross-sectional dimension of the column and the encased steel section size, emerge as key factors influencing the fire performance of high-strength CES columns[3]. Columns with smaller cross-sectional dimensions tend to experience a more rapid temperature rise under fire exposure, whereas larger columns may develop more pronounced internal temperature gradients, indicating that the size effect can affect the fire-induced response and load-bearing evolution of CES columns. Moreover, for a given cross-sectional dimension, variations of the encased steel profile size further modify the thermal response and load-sharing mechanisms between steel profile and concrete. To this end, a coupled thermo-mechanical finite element model is developed to conduct a systematic parametric study on CES columns under fire conditions. The cross-sectional dimension, encased steel section size, material strength, column relative slenderness, and axial load ratio are considered in this study, comprising a total of 405 analysis cases. The results demonstrate that both the cross-sectional dimension and the encased steel profile size alter the temperature development and load-sharing paths between steel profile and concrete under fire, leading to distinct load-bearing mechanisms and significantly influencing the fire resistance of CES columns. The findings of this investigation provide a rational basis for evaluating fire design methods and optimizing the geometric configuration of high-strength CES columns.