Body size is a fundamental functional trait of organisms, influencing spatial use, lifespan, reproductive capacity, and functional roles within ecological networks. In natural ecosystems, geographical variations in insect body size are often explained by classical ecological hypotheses, such as Bergmann's rule (larger sizes in colder climates), the drought adaptation hypothesis (larger sizes in arid environments to reduce water loss), and/ or the seasonality hypothesis (smaller sizes in highly seasonal climates to facilitate rapid reproduction). However, urban ecosystems exhibit distinct microclimates from those in the adjacent natural habitats, characterized by phenomena such as heat islands and altered humidity regimes. A critical, unresolved question is whether these anthropogenic modifications decouple the relationship between regional climate and body size, rendering large-scale geographic patterns unpredictable in urban insect communities. To address this knowledge gap, we conducted a large-scale field study along an extensive elevational gradient (510–3380 m a.s.l.) in the Southwest Mountains at the eastern margin of the Qinghai-Tibet Plateau. We systematically sampled ground-foraging ant communities using pitfall traps from 64 (sub)urban sites across eight counties. A total of 55,890 worker ants, representing 81 species from 31 genera, were collected. Weber's length was measured for each species and used to calculate the community-weighted mean and community mean of body size to characterize the ant assemblages. We employed one-way ANOVA to assess whether the body size of ant communities differed significantly among counties, and performed linear mixed-effects models to test the potential effects of temperature, precipitation and seasonality on the body size variations. We found that body size of ant communities differed significantly among counties (e.g., the average values of community-weighted mean and community mean were approximately 136% and 85% higher in Kangding than those in Yongren, respectively). Consistent with Bergmann's rule, ant communities exhibited larger body sizes in regions with lower annual mean temperatures. In contrast, neither annual precipitation nor precipitation seasonality had significant effects on body size, which does not support the drought adaptation hypothesis or the precipitation-related aspect of the seasonality hypothesis. Contrary to the seasonality hypothesis, however, the community-weighted mean of body size was positively associated with temperature seasonality. Our study highlights that body size in urban ant communities remains sensitive to regional temperature variation, while also suggesting that land management practices (such as irrigation) may mediate the influence of precipitation regimes. Consequently, we propose that monitoring the responses of ant functional traits such as body size to regional climate gradients can serve as a valuable bioindicator tool. This approach shows great promise for broadly assessing the ecological impacts of land disturbance and urbanization, thereby offering fresh insights into the mechanisms sustaining biodiversity in an era of global change.