Although urban nature-based solutions (NBS) generally produce positive results compared to non-NBS interventions, translating these evidence-based approaches into site-specific and measurable applications within urban contexts remains challenging. Key questions remain regarding the optimal number, location, and spatial extent of NBS required to maximize their effectiveness. NBS must possess sufficient size and spatial distribution to support ecological stability and enhance ecosystem services. However, most cities face constraints due to limited available open space. This study investigates how the spatial characteristics of NBS influence their effectiveness in mitigating three selected UCs and examines the relationship between these characteristics and the socio-geographical context. Case studies from Poland (Lublin) and Türkiye (Istanbul) were analyzed across 17 NBS types and 26 indicators. The results indicate that, irrespective of socio-geographical context, urban waters and forests exert the greatest influence on climate resilience. In terms of green space management, the Polish case demonstrated higher overall effectiveness. Nevertheless, the findings did not conclusively demonstrate a direct relationship between NBS spatial structure and biodiversity enhancement. Beyond spatial factors, proximity to UCs, multifunctionality, and protected status emerged as critical determinants of NBS contributions to urban challenges. The evidence generated in this study can inform urban policy and planning by integrating spatial and socio-geographical considerations, thereby optimizing the potential of NBS to effectively mitigate urban challenges.