Radius−luminosity relation for literature reverberation and interferometry size measurements (blue and orange crosses) compared to our new GRAVITY measurements (filled circles). The dashed line is the best-fitting R ∼ L^1/2 fit to reverberation measurements (Suganuma et al. 2006; Koshida et al. 2014). We use FT measurements and our own bolometric luminosity estimates in all cases. The radii reported here for all interferometry measurements is that of a thin ring with an assumed unresolved point-source flux fraction of 20% (see Sect. 5.2 for details).