Definition of the maximum size X. The two sets of curves represent faint (upper) and bright (lower) sources, with an S/N of 10 and 100. Shown are profiles of Gaussian (blue) and power-law (red) sources with a peak intensity I_P = 100 and an FWHM size H = 8′′ on a flat background (green), affected by random Gaussian noise (with σ = 10 and 1). The noise-free profiles are shown by black curves. Dashed horizontal lines (blue and red) visualize zero-level sizes Z that would be estimated for the two sources and the solid horizontal lines (cyan and magenta) show the corresponding X values (assuming η = 3). In the high-S/N case, Z ≈ 24 and 70′′, whereas X = 8 and 23′′. In the low-S/N case, Z ≈ 18 and 26′′, whereas X = 8 and 8.7′′.