When biomass is thermochemically processed, the size of the biomass particles affects processing time requirements and yields. This study investigates the effects of particle size at the centimeter scale on pyrolysis through both experimental and modeling approaches, with three types of woody biomass; poplar, pine sapwood, and pine heartwood. Large (D = 3.81 cm) and small (D = 2.54 cm) wood spheres were pyrolyzed under thermally thick conditions at three final pyrolysis temperatures in a reactor with turbulent gas flow; the same wood materials were also pyrolyzed in a thermogravimetric analyzer (TGA), under kinetic control. The experiments were simulated using a previously published 1-dimensional pyrolysis model, which includes transport and kinetics of solid to vapor reactions for biomass components. Particle size had a strong effect on devolatilization timing and also affected the yields of some species. The model was successful at predicting the qualitative features and approximate magnitudes of quantities such as temperature overshoot, product yields for thermally thick particles, and devolatilization timing in both TGA and thermally thick particles. However, the dependence of yields and timing on wood type and particle size were not reproduced by the model.