  Following the detection of a gravitational microlensing high magnification
event (HME) in Q2237+0305A attempts have been made to place limits on the
dimensions of the quasar continuum source. The analyses have studied either the
observed event magnitude or the event duration. The latter approach has been
hampered by lack of knowledge about the transverse velocity of the lensing
galaxy. We obtain both upper and lower statistical limits on the size of the
continuum source from the observed HME using determinations of transverse
velocity obtained from the published monitoring data. Our calculations take
account of the caustic orientation as well as the component of the caustic
velocity that results from stellar proper motions. Our determination of source
size relies on an estimated duration of 52 days for the HME, and so will be
refined when more HMEs are observed. We find that the upper and lower limits on
the magnified region of the R-band continuum source are 6 times 10^15 and 2
times 10^13 cm respectively (99% confidence). Through consideration of the
joint probability for source size and mean microlens mass we find that the mean
mass lies between ~0.01 solar masses and ~1 solar mass (95% confidence).
