  We report on phase-referenced VLBI radio observations of the Type IIb
supernova 2011dh, at times t = 83 days and 179 days after the explosion and at
frequencies, respectively, of 22.2 and 8.4 GHz. We detected SN 2011dh at both
epochs. At the first epoch only an upper limit on SN 2011dh's angular size was
obtained, but at the second epoch, we determine the angular radius SN 2011dh's
radio emission to be 0.25 +- 0.08 mas by fitting a spherical shell model
directly to the visibility measurements. At a distance of 8.4 Mpc this angular
radius corresponds to a time-averaged (since t=0) expansion velocity of the
forward shock of 21000 +- 7000 km/s. Our measured values of the radius of the
emission region are in excellent agreement with those derived from fitting
synchrotron self-absorbed models to the radio spectral energy distribution,
providing strong confirmation for the latter method of estimating the radius.
We find that SN 2011dh's radius evolves in a power-law fashion, with R
proportional to t^(0.92 +- 0.10).
