1. The relationship between plant size and trait characteristics is a fundamental aspect of the global spectrum that encompasses plant form and function. However, it remains unclear whether plant size affects interspecific variation in leaf structural traits at a specific ontogenetic stage. 2. In this research, we investigated the leaf structural traits, plant height, and diameter at breast height (or basal diameter) of 10 shrub and tree species. This study was carried out along an intertidal gradient within a mangrove forest located in Southeast China. 3. We found that leaf traits differed significantly between shrubs and trees in their response to intertidal gradients, which contributes to the advancement of our understanding of plant adaptive strategies and the evolution of traits within varying environmental contexts. Among all species, leaf carbon economics (leaf dry mass content, leaf mass per area, and leaf density) decreased significantly with increasing plant height and diameter. For each growth form and intertidal zone, the plant size-trait patterns were consistent with those in the pooled dataset. 4. Collectively, these findings suggest that mangrove plants undergo a size-dependent shift from resource conservation to resource acquisition strategies with increasing stature. Therefore, plant size serves as an indicator of the “slow-fast” spectrum of plant performance and the dichotomy between conservation and acquisition strategies.