Anal Training Collection
Anal training is key for teaching your sphincter muscles to stretch comfortably. You'll want to stretch slowly, and stop if you feel any pain. Remember to always use lubricant with anal play. The ideal step up is 0.33 inches diameter (or 1 inch circumference) or less, between a toy you can fit and the next size up!
See this article for more detail on How to Size Up with sex toys.