For my AN, depending on who does the measurement I have heard sizes ranging from 2.4 cm (according to the radiologist report), to a whopping 3.5 cm (from one of the doctors I consulted with). In a separate thread Tumbleweed posted a very nice explanation about how the measurements can vary, depending on if they are taken on an oblique angle, etc. Also, some doctors report the largest dimension (including the tiny portion in the IAC) and some only report the true diameter of the approximately spherical portion in the CPA. In my opinion this is the explanation for the big discrepancies reported on the MRI and after surgical removal: For example the radiologist may report only the diameter of the spherical portion, and upon removal the surgeon reports the largest dimension, which can easily be 0.5-1 cm longer.
After all this I am convinced that the most reliable way to be sure is to look at my own scans and compare my own measurements to the previous ones.
My understanding is that for radiation, the only dimension that matters is the diameter of the spherical portion, as this determines the total volume that must be radiated.
Marianna