We investigate the effect of crystal size on the rheology of basaltic magmas by means of a rheometer and suspensions of silicon oil with natural magmatic crystals of variable size (from 63 to 0.5 mm) and volume fraction ϕ (from 0.03 to 0.6). At constant ϕ, finer suspensions display higher viscosities than coarser ones. Shear thinning (flow index n < 1) occurs at ϕ > 0.1–0.2 and is more pronounced (stronger departure from the Newtonian behavior) in finer suspensions. Maximum packing and average crystal size displays a nonlinear, positive correlation, while yield stress develops at ϕ > 0.2–0.3 irrespective of the crystal size. We incorporate our results into physical models for flow of lava and show that, with respect to lava flows containing coarser crystals, those with smaller crystals are expected to: 1) flow at lower velocity, 2) have a lower velocity gradient, and 3) be more prone to develop a region of plug flow. Our experimental results explain the observation that phenocryst-bearing and microlite-bearing lavas at Etna volcano (Italy) show smooth pahoehoe and rough aa' surfaces, respectively.