Covalent organic frameworks (COFs) consist of monomers arranged in predictable structures with emergent properties. However, improved crystallinity, porosity, and solution processability remain major challenges. To this end, colloidal COF nanoparticles are useful for mechanistic studies of nucleation and growth and enable advanced spectroscopy and solution processing of thin films. Here we present a general approach to synthesize imine-linked 2D COF nanoparticles and control their size by favoring imine polymerization while preventing the nucleation of new particles. The method yields uniform, crystalline, and high-surface-area particles and is applicable to several imine-linked COFs. In situ X-ray scattering experiments reveal the nucleation of amorphous polymers, which crystallize via imine exchange processes during and after particle growth, consistent with previous mechanistic studies of imine-linked COF powders. The separation of particle formation and growth processes offers control of particle size and may enable further improvements in crystallinity in the future.