I designed some 6 pointed stars using a construction circle of 45mm diameter and thought when I sliced and 3D printed they would be 45mm across each pair of points. They were not. I measured the 3D model in CAD and got 45mm and when I looked at the width of the part in Prusa Slicer it showed 45mm but it was printing 43mm.
I even brought up my 40x40x40mm calibration cube and it measured exactly 40x40 in X and Y.
Instead of rounding off the points of the stars I just added 2mm to my construction circle diameter and it now prints close enough to 45mm.
BTW, the inside diameter of toilet paper rolls is ~45mm. 
This project is to help ensure the TP rolls are mostly round when I put them on my laser cutter rotary tool.