Colloidal semiconductor quantum dots (QDs) have been well appreciated for their potential in nanophotonics with an unprecedented impact in various areas, including light emitting diodes (LEDs) and solar cells. There is an outstanding demand on the control of size and size distribution for the various applications, with rational design supported by fundamental understanding of nucleation and growth. This Research News introduces recent advances in the synthesis of colloidal CdSe magic-sized nuclei (MSN) exhibiting sharp bandgap emission, with a model proposed to illustrate the nature of monomers and their degree of supersaturation (DS) affecting the formation of various CdSe MSN, magic-sized nanoclusters (MSCs), and regular nanocrystals (RNCs). Also, this model addresses tuning the CdSe RNCs into the CdSe MSN with the presence of cadmium acetate (Cd(OAc)2) affecting the nature of the monomers.