Urban heat islands pose major challenges to public health and sustainable urban development under intensified climate change and urbanization. Vegetation plays a key role in mitigating urban heat islands, yet its cooling effectiveness is jointly influenced by meteorological conditions, plant physiology, and anthropogenic factors. Leaf size is one of the key factors influencing the energy exchange between vegetation and the atmosphere. However, quantitative research on its impact on the cooling effect of vegetation remains limited, particularly in humid urban environments where leaf sizes exhibit significant variation. This study focuses on Nanjing City, employing a coupled canopy energy balance model and localized stomatal conductance model to quantitatively assess differences in sensible heat flux under typical summer conditions for three tree species—needleleaf, small broadleaf, and large broadleaf—under varying meteorological conditions, using an approximate leaf area index. Results indicate that the model-simulated canopy temperature consistently aligns well with satellite-observed surface temperature throughout the year (R2>0.89)；Compared to large broadleaf vegetation, small broadleaf vegetation produces lower canopy temperatures and sensible heat flux under reduced boundary layer resistance, thereby exhibiting a stronger cooling effect per unit leaf area. Although needleleaf vegetation possess the lowest boundary layer resistance, their sensible heat flux is highest among the three vegetation types due to constraints imposed by low stomatal conductance, resulting in the weakest cooling effect. Heat flux and cooling differences are simultaneously influenced by meteorological conditions, exhibiting greater variation under humid and cloudy conditions while diminishing under conditions of high radiation and high saturated vapor pressure. When other meteorological conditions are similar, increased wind speed can amplify heat flux differences among various vegetation types. Therefore, selecting small broadleaf tree species in well-ventilated neighborhoods holds promise for significantly improving the thermal environment. The findings of this study provide theoretical support for optimizing green space allocation in thermal environment retrofits of humid urban old districts and new urban development in China.