The use of sterically hindered N,N′-diisopropylbenzene-1,2-diamine (DIBA) for aminophosphine-based InP QDs synthesis in an oleylamine (OLAm) medium is evidenced as a powerful tool to finely control their size in a reproducible way. This diamine forms in situ a P(DIBA)(OLAm) species accounting for the observed size-tuning effect because the QDs crystallize as tetrahedra with an edge length (3.5 to 6.6 nm) depending on the DIBA concentration (0 to 7 eq./indium). This kinetically controlled size-tuning mechanism is proposed by combining 31P NMR and UV–vis spectroscopy. A proof-of-concept is established by coating the InP cores with ZnS shells. The luminescence of the as-synthesized InP/ZnS QDs is tunable over the 480–620 nm range, with photoluminescence internal quantum efficiencies of 45 to 60% suitable for applications as phosphors in LED lighting systems and LED displays.