Cities worldwide are increasingly adopting green interventions, such as nature-based solutions(NBS), to meet urban sustainability goals. However, how and where to optimize investments to receive the most benefit for NBS implementation are largely unknown. Here, we provide a continental-scale assessment of the effect of urban green areas for cooling across 32 Chinese cities that vary in size, climate, and native biome. The relationship between green patch size and surface temperature reveals a consistent decreasing concave-up relationship across varied cities, indicating that increasing green patch size initially corresponds to a decrease in temperature. However, this pattern shifts such that cooling benefits decrease per area once patch size increases to a specific climate threshold, a green-area threshold (GAT) for cooling. The direct implication is that large green space patches will be needed in temperate and drier cities for effective cooling as a climate adaptation strategy, whereas a smaller threshold is recommended in the tropics.