  Precise stellar masses and radii can be determined using asteroseismology,
but their accuracy must be tested against independent estimates. Using radii
derived from Gaia DR2 parallaxes, we test the accuracy of asteroseismic radii
for a sample of 93 dwarfs based on both individual frequency fitting and the
seismic scaling relations. Radii from frequency fitting are about 1 per cent
smaller than Gaia radii on average; however, this difference may be explained
by a negative bias of 30 $\mu$as in the Gaia parallaxes. This indicates that
the radii derived from frequency fitting are accurate to within 1 per cent. The
scaling relations are found to overestimate radii by more than 5 per cent,
compared to the Gaia radii, at the highest temperatures. We demonstrate that
this offset is reduced to 3 per cent after applying corrections based on model
frequencies to the scaling relation for $\Delta\nu$, but only when the model
frequencies are corrected for the surface effect. With corrections to
$\Delta\nu$, the scaling relation gives radii accurate to about 2--3 per cent
for dwarfs in the temperature range $5400$--$6700$ K. The remaining offset at
the highest temperatures may indicate the need for a correction to the scaling
relation for $\nu_{\mathrm{max}}$.
