HMMM>..your surgery was shortly after mine. It sounds to me as if you had a blockage and when the blockage worked its way out, the stoma shrunk in size. I never had a major blockage, but there were a few times when food seemed to get "stuck" so to speak, and my stoma became swollen. I never changed my wafer size or anything like that.
I have been using the Coloplast Sensura one piece bags with the convex wafer for over two years. My stoma always has a little redness around it but it never bothers me. Some days are worse than others depending on whether I eat spicy foods. The stoma powder usually helps.
If I notice my output is unusually thick, I immediately drink a LOT of water. It seems to do the trick.
Now that you mentioned it, my stoma seems to change in shape from day to day, but as long as it is working and doesn't give me grief...I don't worry about it.