Spatial measurements in Oblique plane of Anisotropic Volume

Hello, I am working with 3D CTA data for vessel geometry quantification . Depending upon centreline points, I have extracted oblique plane from 3D volume and now I want to quantify the vessel shape. My problem is to make accurate measurements for diameter & area of vessel.

Since the 3D CTA volume is Anisotropic having voxel size (0.25mm*0.25mm*0.40mm) , I am not sure what will be pixel size (length*height) in extracted cross section/oblique plane. In order to record measurement, I need to know spatial calibration factor, but as This 2D image has been extracted from 3D volume using interpolation, I am not sure about pixel size. For instance, If Oblique plane gives me lumen radius equal to 30 pixels, now what should I use (calibration factor) to convert this 30 unit distance into millimeter scale?

In case of an ideal ISOTROPIC volume, it would have been easy as oblique plane become independent of voxel size in 3D (for instance all 3 dimensions having voxel size 0.4mm*0.4mm*0.4mm)but Its getting difficult in my case.

Any help will be greatly appreciated