Urban road dust can be enriched in different elements and hence it can pose a threat to human and environmental health. Proximal soil sensing allows the swift monitoring of such particles in order to drive attention to any possible risks. The goal of this study is to find the variation in concentrations found when using a portable X-ray fluorescence (XRF) proximal sensor for the determination of metals in road dust as the particle size of the samples decreases. In general, readings decrease as the particles are larger. The maximum reading occurs when particles are sieved to 100 µm but a particle size of 250 µm maintain a 90% accuracy. This study can help field workers to define an expected accuracy when using such type of sensors in street dust analysis.