Aims: We derive the thermal inertia of 2008 EV5, the baseline target for the Marco Polo-R mission proposal, and infer information about the size of the particles on its surface. Methods: Values of thermal inertia were obtained by fitting an asteroid thermophysical model to NASA's Wide-field Infrared Survey Explorer (WISE) infrared data. Grain size was derived from the constrained thermal inertia and a model of heat conductivity that accounts for different values of the packing fraction (a measure of the degree of compaction of the regolith particles). Results: We obtain an effective diameter D = 370 ± 6 m, geometric visible albedo pV = 0.13 ± 0.05 (assuming H = 20.0 ± 0.4), and thermal inertia Γ = 450 ± 60 J m-2 s-1/2 K-1 at the 1σ level of significance for its retrograde spin-pole solution. The regolith particles radius is r = 6.6+1.3-1.3 mm for low degrees of compaction and r = 12.5+2.7-2.6 mm for the highest packing densities. Table of observed fluxes is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/561/A45