Just choose the diameter of the hole that you want to cut (3" recommended) enough to connect your wiring and hang your light fixture or ceiling fan, then after finding the exact center point in your medallion, use a tool called hole saw that can be easily found in a local store to drill the hole.
Some medallions are not appropriate for canopies, in this type of medallions is recommended not to use a canopy, just drill a hole the light fixture chain diameter and leave the medallion decorative center to work as your chain base or canopy if you want.
In this case, you have a combination of factors, first your electric box, which usually is recommended cutting 3 inches, and secondly your light fixture or ceiling fan canopy diameter. Both factors should be considered before choosing hole size, as you need room for all your wiring and hanging parts, and at the same time this hole should be covered by your canopy, also view product attribute to see what is this ceiling medallion canopy fitting specification. Please keep in mind the ceiling medallions with center hole cut are not returnable, because they are not resalable.