  We show that observations of X-ray pulsing from SAX J1808.4-3658 place a firm
upper limit of 13.8 m^{1/3} km on the radius of the neutron star, where m is
its mass in solar units. The limit is independent of distance or assumptions
about the magnetospheric geometry, and could be significantly tightened by
observations of the pulsations in the near future. We discuss the implications
for the equation of state and the possible neutron star mass.
