The agrifood industries generate tremendous amounts of waste. The valorisation of these wastes is of the utmost importance. Here, spent coffee ground (SCG) and Cistus ladanifer L. leaf (CLL) post-distillation residues were used to prepare 50:50 (v/v) hydromethanolic extracts for green zero-valent iron nanoparticle (nZVI) production. Then, the nZVIs’ size, polydispersity index (PDI) and zeta potential (ZP) were determined through dynamic light scattering (DLS). Since nZVIs are known to be heavily reactive and display a tendency to agglomerate, dispersant influence (water or methanol) and surfactant addition (Tween® 20) were studied. SCG NPs dispersed in water displayed a size of 565.6 ± 80.84 nm, with a PDI of ± 0.084, and a ZP of -19.57 ± 0.95 mV. Adding Tween®-20 resulted in much lower sizes for these NPs (14.64 ± 0.76 nm with a PDI of 0.238 ± 0.066) and an increase in ZP (-5.99 ± 1.71 mV). CLL nZVIs dispersed in water displayed similar results, with lower size and higher ZP after surfactant addition (766.43 ± 129.49 nm, 0.684 ± 0.151 PDI vs. 13.4 ± 4.26 nm, 0.31 ± 0.042 PDI, -5.51 ± 0.86 mV). Using methanol as the dispersant for nZVIs displayed far worse results, which shows that nZVIs are better dispersed in water, and the addition of Tween® 20 highly reduced agglomeration, increasing the zeta potential. These results allow for better understanding of the importance of dispersant and surfactant usage for an accurate characterization by DLS.