Various concentrations of Matricaria chamomilla (M. chamomilla) flower extract was used for biosynthesis of Ag NPs with different sizes, 70 +/- 5 (Ag NP-1), 52 +/- 5 (Ag NP-i) and 37 +/- 4 nm (Ag NP-3), and size-specific antimicrobial activity of them was evaluated towards Gram + Staphylococcus aureus (S. aureus), Gram Escherichia colt (E. colt) bacteria and Candida fungus albicans (C. albicans). We also examine the formation of Ag NPs as a function of the plant extract volume, Ag ion concentration (Ag+) and reaction time. M. chamomilla flower extract at 100 ppm shows mild inhibitory effect by inhibiting growth of all target pathogens. The extract mediated Ag NPs even at low concentration (6.25 ppm) caused reductions in cell densities of the pathogens. The potential reasons for the highly effective inhibitory activity of the extract mediated Ag NPs are: (1) synergistic effects due to combination of M. chamomile extract and Ag NP because Ag NP is an effective germicidal and M. chamomile plant itself shows mild inhibitory property, (2) high localized concentration of M. chamomile extract due to nano scale entrapment of it on surface of Ag NP and (3) size-specific antibacterial efficacy of Ag NP because small size increases the active surface area of Ag NP, which reacting bacterial cells and increases number of extract molecules anchored on the surface of Ag NP.