This review discusses the impact of nanoparticle (NP) size on their interactions with cells, emphasizing the effects on cellular uptake, intracellular localization, and cytotoxicity. The study highlights the importance of NP size in determining their biological behavior, including their ability to enter cells through endocytosis or passive penetration. It reviews various techniques for characterizing NP size, such as transmission electron microscopy (TEM), dynamic light scattering (DLS), fluorescence correlation spectroscopy (FCS), and nanoparticle tracking analysis (NTA). The review also discusses the formation of the protein corona around NPs, which influences their biological interactions. NP size affects the mechanisms of cellular uptake, with smaller NPs often being internalized via receptor-mediated endocytosis, while larger NPs may be taken up through phagocytosis or passive diffusion. The size of NPs also influences their intracellular distribution and the potential for cytotoxic effects. Smaller NPs have a higher surface area-to-volume ratio, which can increase their reactivity and potential for causing cellular damage. The review also addresses the challenges in accurately assessing NP toxicity, as in vitro results may not always translate to in vivo effects. The study concludes that NP size is a critical factor in determining their biological interactions, and that understanding these interactions is essential for the safe and effective design of nanomaterials for biomedical applications. The review emphasizes the need for further research to better understand the complex interactions between NPs and biological systems, and to develop more accurate methods for assessing NP toxicity and cellular uptake.This review discusses the impact of nanoparticle (NP) size on their interactions with cells, emphasizing the effects on cellular uptake, intracellular localization, and cytotoxicity. The study highlights the importance of NP size in determining their biological behavior, including their ability to enter cells through endocytosis or passive penetration. It reviews various techniques for characterizing NP size, such as transmission electron microscopy (TEM), dynamic light scattering (DLS), fluorescence correlation spectroscopy (FCS), and nanoparticle tracking analysis (NTA). The review also discusses the formation of the protein corona around NPs, which influences their biological interactions. NP size affects the mechanisms of cellular uptake, with smaller NPs often being internalized via receptor-mediated endocytosis, while larger NPs may be taken up through phagocytosis or passive diffusion. The size of NPs also influences their intracellular distribution and the potential for cytotoxic effects. Smaller NPs have a higher surface area-to-volume ratio, which can increase their reactivity and potential for causing cellular damage. The review also addresses the challenges in accurately assessing NP toxicity, as in vitro results may not always translate to in vivo effects. The study concludes that NP size is a critical factor in determining their biological interactions, and that understanding these interactions is essential for the safe and effective design of nanomaterials for biomedical applications. The review emphasizes the need for further research to better understand the complex interactions between NPs and biological systems, and to develop more accurate methods for assessing NP toxicity and cellular uptake.