•FACS is a useful tool to separate soil microbes of different sizes.•Different cell size classes harbor distinct bacteria communities.•Bacterial core community varies according to size classes and soil type.•Bacterial functional similarities of different size classes depend on soil type. Cell size is a key morphological trait which is associated with microbial activity and nutrient acquisition. However, it is still unclear whether bacteria of different sizes have similar structural and functional properties. In this study, we sorted bacterial cells into five size classes (small, slightly small, medium, slightly large, large) using FACS and compared their structural and functional profiles in soils from deciduous and evergreen forests. The results showed that most (about 60%) of the bacterial cells fell under small or slightly small size classes. The five size classes harbored distinct bacterial communities in both types of forest soil. In spite of the lower relative abundance, slightly large and large bacterial cells had higher diversity compared to other size classes. Core communities of the five size classes in evergreen forest soil harbored more ubiquitous OTUs when compared with deciduous forest soil. Bacterial functional structures of the five size classes were significantly different in deciduous forest soil, while similar across size classes in evergreen forest soil. We conclude that cell size is an important factor that determines bacterial structure and function, and this relationship is depended on ecosystem or soil type. This study emphasizes cell size as a useful tool when assessing bacterial diversity and uncovers a direct relationship between microbial morphology and ecological traits.