You can purchase a stoma cap in a variety of sizes. They are pre-made to coordinate with the corrdinating ostomy supplies. You can purchase the size stoma cap that is most useful for your needs. Depending on your typical bowel movements, you may want the bag to be bigger or smaller. This can be a very individual selection. However, stoma caps are very discrete and thin so they don't actually hold very much waste. You can also cut your stoma cap to the size that you need.