The zipper mechanism explains phagocytosis as sequential ligand–receptor interactions between macrophages and antigens, followed by the cell membrane extension for phagosome formation. Hence, the ability of macrophages to innate immunity is restricted by their capacity for engulfment related to the expansion limit of their membranes. However, the maximum expansion capacity of macrophage membranes and its relationship with phagocytic capacity have not yet been rigorously investigated due to the lack of quantitative methods for measuring maximum cell expansion. Here, we developed an opsonized capillary tube assay and evaluated the maximum membrane extension on the opsonized inner surface of capillary tubes, measured as the distance from the inner round area to the macrophage surface. When the engulfment started, the cell membrane in the inner circle of the capillary head expanded up to 10.64 times in opsonized capillary tubes regardless of the inner diameter differences of 3 to 7 µm. This maximum expansion capacity was twice that reported in the frustrated phagocytosis experiments. To support this result, we confirmed the independence of simultaneous local phagocytic responses against multiple antigens and the phagocytic ability of the outer surfaces of extending phagocytic cups. We applied this maximum expansion capacity to opsonized microneedle phagocytosis and estimated that the required cell membrane area for phagocytosis to reach the maximum expansion was up to 5.05 µm around the attached antigen. The maximum number of engulfed 40 µm microbeads during serial phagocytosis was 24% of their maximum ability to extend membranes, suggesting that serial phagocytosis may involve another phagocytosis-number-dependent regulatory mechanism, in addition to the zipper mechanism, to explain the determination of phagocytosis capacity.