Check the size that's marked on the current ones, measure them to see if that number is accurate, then squish them under a couple of heavy books or a clamp for a couple of hours. Re-measure the squished size, then give them a try before they rebound to see if the squished size is in the right ballpark.
If the plastic backing of the pads has holes, you can also snip away and pull out some of the foam from the back with long narrow scissors. I do this with mine to relieve a sharp pressure point on my ears from the model that I wear.