The impact of small-scale mixing and scalar dissipation rate (SDR) fluctuations on the evolution of soot particle size distribution (PSD) dynamics in non-premixed turbulent flames is investigated within the context of Large Eddy Simulation (LES). To accomplish this, a stochastic approach based on a physicochemical sectional model, spatially homogeneous Conditional Moment Closure (CMC) and a stochastic differential equation for the SDR are employed, mimicking the function of an LES sub-grid scale combustion model. The results reveal the influence of SDR fluctuations on the probability density function (PDF) and the dynamic behaviour of gas-phase species and soot PSD moments, emphasising the importance of micromixing, the LES resolution, and the underlying assumptions regarding particle transport. This study provides a framework for better understanding the effectiveness of presumed PDF sub-grid scale combustion models in describing soot evolution.