Highly-monodisperse biocompatible and functionalizable sub-10 nm citrate-stabilized gold nanoparticles (Au NPs) have been synthesized following a kinetically controlled seeded-growth strategy. The use of traces of tannic acid together with an excess of sodium citrate during nucleation is fundamental in the formation of a high number (7·1013 NP/mL) of small ~3.5 nm seeds with a very narrow distribution. A homogeneous nanometric growth of these seeds is then achieved by adjusting the reaction parameters: pH, temperature, sodium citrate concentration and gold precursor to seeds ratio. We use this method to produce Au NPs with a precise control over their sizes between 3.5 and 10 nm and a versatile surface chemistry allowing studying the size-dependent optical properties in this transition size regime lying between clusters and nanoparticles. Interestingly, an inflexion point is observed for NPs smaller than 8 nm in which the sensitivity of the Localized Surface Plasmon Resonance (LSPR) peak position as a function of NPs size and surface modifications dramatically increased. These studies are relevant in the design of the final selectivity, activity and compatibility of Au NPs, especially in those (bio)applications where size is a critical parameter (e.g. biodistribution, multiplex labeling and protein interaction).