  We report on third epoch VLBI observations of the radio-bright supernova SN
2011dh located in the nearby (7.8 Mpc) galaxy M51. The observations took place
at $t=453$ d after the explosion and at a frequency of 8.4 GHz. We obtained a
fairly well resolved image of the shell of SN 2011dh, making it one of only six
recent supernovae for which resolved images of the ejecta are available. SN
2011dh has a relatively clear shell morphology, being almost circular in
outline, although there may be some asymmetry in brightness around the ridge.
By fitting a spherical shell model directly to the visibility measurements we
determine the angular radius of SN 2011dh's radio emission to be $636 \pm 29$
$\mu$as. At a distance of 7.8 Mpc, this angular radius corresponds to a linear
radius of $(7.4 \pm 0.3) \times 10^{16}$ cm and an average expansion velocity
since the explosion of $19000^{+2800}_{-2400}$ kms$^{-1}$. We combine our VLBI
measurements of SN 2011dh's radius with values determined from the radio
spectral energy distribution under the assumption of a
synchrotron-self-absorbed spectrum, and find all the radii are consistent with
a power-law evolution, with $R \sim t^{0.97\pm0.01}$, implying almost free
expansion over the period $t=4$ d to 453 d.
