UiO-66, a zirconium­(IV) metal–organic framework composed of six-metal clusters and terephthalic acid ligands, displays excellent thermal and chemical stability and has functions in gas storage, catalysis, selective adsorption, and drug delivery. Though the stability of UiO-66 is highly advantageous, simultaneous synthetic control over particle size and defectiveness of UiO-66 remains difficult to attain. Using an acid-free solvothermal synthesis, we demonstrate that particle size, defectiveness, and inherent fluorescence of UiO-66 can be precisely tuned using the molar ligand-to-metal ratio, quantified water content, and reaction time during synthesis. These three synthetic handles allow for reproducible modulation of UiO-66 defectiveness between 0 and 12% and particle size between 20 and 120 nm, while maintaining high crystallinity in the nanoparticles that were formed. We also find that particle defectiveness is linked to common overestimation of particle size measurements obtained via dynamic light scattering and propose a model to correct elevated hydrodynamic diameter measurements. Finally, we report inherent fluorescence of nonfunctionalized UiO-66, which exhibits peak fluorescence at a wavelength of 390 nm following excitation at 280 nm and is maximized in large, defect-free particles. Overall, this synthetic approach and characterization of defect, size, and fluorescence represent new opportunities to tune the physiochemical properties of UiO-66.