PDF of the source size for a standard accretion disc model (ν = 4/3) and various fractions of compact objects f_∗ (extreme values f_∗ = 1, shown in light gray, and f_∗ = 0.01, shown in dark gray, correspond to 100% and 1% of the total matter being compact, respectively). The thick blue line is the result obtained assuming the same prior on f_∗, as in Fig. 5, namely the prior resulting from the analysis of Pooley et al. (2012). The vertical solid line is the theory size for a Shakura-Sunyaev model based on Eq. (10) for cos(i) = 0.5 (R_1/2 ~ 3.04 × 10¹⁵ cm ~10^-3 pc).