We present photometric observations of four transits in the WASP-17 planetary system, obtained using telescope defocusing techniques and with scatters reaching 0.5mmag per point. Our revised orbital period is 4.0+/-0.6s 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+/-0.010R_Jup_ (statistical and systematic errors, respectively), provides confirmation that WASP-17b is the largest planet currently known. All 14 planets with radii measured to be greater than 1.6R_Jup_are found around comparatively hot (T_eff_>5900K) and massive (M_A_>1.15M_{sun}_) 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.