The accurate monitoring of particulate emissions from medium-scale, decentralized biomass combustion units is a major challenge for the deployment of this technology in the framework of the current energy transition. More specifically, the experimental characterization of the size of the emitted particles, i.e., using impactors, is still subject to discussions about the impact of the methodology on the measurement results. To meet these challenges, particulate emissions from a medium-scale biomass boiler (4.5 MWth) were measured with the Electrical Low Pressure Impactor (ELPI+) using two different dilution systems—one containing a single stage and the other, dual stages—to examine the effect of dilution and the performance of the ELPI+. No statistically significant correlation was found with either system between the dilution ratio (DR) and the total particle number concentration, Ntot, or between the DR and the mass concentration, mtot. However, a significant positive correlation was observed with both systems between the DR and the concentration of particles with a diameter, Dp, below 0.01 µm due to measurement artefacts. With the one-stage dilution system, the condensation appeared to be enhanced when the DR was reduced. When the ELPI+ impactor was not overloaded, the number concentrations of the N<0.01 and N0.01-0.1 size fractions decreased over time due to the premature measurement of particles. However, when the impactor was overloaded, the N<0.01 concentration was overestimated, whereas the N0.01-0.1 and N0.1-1 concentrations were underestimated, due to the change in the cut-off diameter.