Maps of source size parameter obtained by fitting CH₃CN (12_K − 11_K) K = 0 – 6 and CH₃¹³CN (12_K − 11_K) K = 0 – 3 lines with XCLASS for the best disk candidates in the CORE survey. The contours correspond to the 1.37 mm continuum as described in Fig. 1. For NGC7538 IRS1, only the region outside the continuum to the south-west is modelled by XCLASS as the molecular lines are seen in absorption against the strong continuum source with many velocity components due to the complex dynamics in this source. The synthesised beam is shown in the bottom left corner and a scale bar in the bottom right corner of each panel.