Halide perovskite nanocrystals are promising materials for optoelectronic applications. Metal doping provides an avenue to boost their performance further, e.g., by enhancing light emission, or to provide additional functionalities, such as nanoscale magnetism and polarization control. However, the synthesis of widely sizetunable nanocrystals with controlled doping levels has been inaccessible using traditional hot injection synthesis, preventing systematic studies on dopant effects toward device applications. Here, we report a versatile synthesis method for metal-doped perovskite nanocrystals with precise control over size and doping concentration under ambient conditions. Our room temperature approach results in fully size-tunable isovalent doping of CsPbX 3 nanocrystals (X = Cl, Br, I) with various transition metals M 2+ tested (M = Mn, Ni, Zn). This gives for the first time access to small, yet precisely doped quantum dots beyond the weak confinement regime reported so far. It also enables a comparative study of the photophysics across multiple size and dopant regimes, where we show dopant-induced localization to dominate over quantum confinement effects. This generalizable, facile synthesis method thus provides a toolbox for engineering perovskite nanocrystals toward light-emitting technologies under industrially relevant conditions.