  We present photometric observations of four transits in the WASP-17 planetary
system, obtained using telescope defocussing techniques and with scatters
reaching 0.5 mmag per point. Our revised orbital period is 4.0 +/- 0.6 s longer
than previous measurements, a difference of 6.6 sigma, and does not support the
published detections of orbital eccentricity in this system. We model the light
curves using the JKTEBOP code and calculate the physical properties of the
system by recourse to five sets of theoretical stellar model predictions. The
resulting planetary radius, Rb = 1.932 +/- 0.052 +/- 0.010 Rjup (statistical
and systematic errors respectively), provides confirmation that WASP-17b is the
largest planet currently known. All fourteen planets with radii measured to be
greater than 1.6 Rjup are found around comparatively hot (Teff > 5900 K) and
massive (MA > 1.15 Msun) stars. Chromospheric activity indicators are available
for eight of these stars, and all imply a low activity level. The planets have
small or zero orbital eccentricities, so tidal effects struggle to explain
their large radii. The observed dearth of large planets around small stars may
be natural but could also be due to observational biases against deep transits,
if these are mistakenly labelled as false positives and so not followed up.
