Measured size distribution of galaxies in the central projected R₂₀₀ for the GAMA groups. Grey pluses: average raw counts per group. Grey note: average values expected from fore- and background interlopers. Grey diamond: background-subtracted values. Black data points: background-subtracted values, corrected for incompleteness, including bootstrap resampling errors. Blue and red points: measured sizes for galaxies split over two bins of mean effective surface brightness, also after correction. In this size range, the data are well described by a power-law index of −2.71 ± 0.33 (in bins of equal logarithmic size). Grey dashed line: size distribution with power-law index of −3.40, which we found for UDGs in clusters (vdB16), arbitrarily normalised.