A series of ZnX2O4:Er3+/Yb3+ (X = Al3+, Ga3+, In3+) nanoparticles were obtained by using a modified Pechini technique. The crystal structure, morphology, and chemical composition of the engineered compounds were characterized in detail by means of X-ray diffraction, Micro-Raman spectroscopy, transmission electron microscopy (TEM) and selected area electron diffraction, and energy-dispersive X-ray spectroscopy. Particle sizes were estimated by using theoretical calculations (Scherrer and Rietveld methods) as well as experimental techniques [TEM and dynamic light scattering (DLS)] and were, furthermore, cross-compared. The calculated average grain size, for dry powders, is in the range 20–45 nm. The hydrodynamic size was measured by using the DLS technique on nonmodified, surfactant-free particles of the whole ZnX2O4 series. The up-conversion properties of the particles were discussed in detail. The low value of the green-to-red ratio points to a significant contribution of non-radiative pathways, such as multiphonon relaxation or cross relaxation processes, to the depopulation of the green band, which feeds and favors the red emission of Er3+ in the up-conversion process.