Display omitted •GO possesses intrinsic antibacterial activity to various bacterial pathogens.•A parabolic relation exists between the size and antibacterial activity of GO.•Increasing GO size enhances cell entrapment effect but reduces cutting effect.•Decreasing GO size reduces cell entrapment effect but enhances cutting effect. The antibiotics-independent antimicrobial activity of graphene oxide (GO) is of great importance since antibiotic therapy is facing great challenges from drug resistance. However, the relations of GO size with its antimicrobial activity and how the size regulates the antibacterial mechanisms are still unknown. Herein, we fabricated four GO suspensions with different sizes and demonstrated the parabolic relationship between GO size and its antibacterial activity against the Gram-positive cariogenic bacterium Streptococcus mutans. More interestingly, we found out how GO size regulated the nano-bio interaction-based physical antibacterial mechanisms. Increasing the size reduced the cutting effect but enhanced the cell entrapment effect, and vice versa. In conclusion, GO size affects its edge density and lateral dimension, further regulates its physical antibacterial mechanisms in different orientations and ultimately determines its activity. These findings provide a deep understanding of GO antibacterial property and may guide the design and development of GO for clinical use.