Estimated demagnified sizes, r_s,demag, for the ALMA-detected sources in the three FFs clusters studied here, as measured by fits in the uv-plane, plotted alongside measured sizes for several literature samples obtained using interferometric data (Younger et al. 2008, 2010; Valtchanov et al. 2011; Rybak et al. 2015; Simpson et al. 2015; Bussmann et al. 2015). The estimated demagnified 1.1 mm flux densities are shown; for the literature sample, when necessary we have converted to this wavelength assuming a slope of β = 1.8. We see that the ALMA-FFs sources studied here exhibit a large dispersion in extent compared to brighter flux density sources; notably ~50% of the ALMA-FFs sources appear comparable to the largest literature sample sources. The dashed line shows the best fit regression to all samples, with log r_s,demag = m × log F_{1.1 mm} + b with m = −0.08 ± 0.10 and b = −0.95 ± 0.05. The orange shaded region shows the 2σ range of possible fits to the data. The measured sizes show no obvious trend with flux density.