Membrane interactions are critical for the successful use of inorganic nanoparticles as antimicrobial agents and as carriers of, or co-actives with, antimicrobial peptides (AMPs). In order to contribute to an increased understanding of these, we here investigate effects of particle size (42-208 nm) on layered double hydroxide (LDH) interactions with both bacteria-mimicking and mammalian-mimicking lipid membranes. LDH binding to bacteria-mimicking membranes, extraction of anionic lipids, as well as resulting membrane destabilization, was found to increase with decreasing particle size, also translating into size-dependent synergistic effects with the antimicrobial peptide LL-37. Due to strong interactions with anionic lipopolysaccharide and peptidoglycan layers, direct membrane disruption of both Gram-negative and Gram-positive bacteria is suppressed. However, LDH nanoparticles cause size-dependent charge reversal and resulting flocculation of both liposomes and bacteria, which may provide a mechanism for bacterial confinement or clearance. Taken together, these findings demonstrate a set of previously unknown behaviors, including synergistic membrane destabilization and dual confinement/killing of bacteria through combined LDH/AMP exposure, of potential therapeutic interest.