Go for the tiniest, or thinnest, if you're just starting to discover the joys of anal play. It is entirely up to you whether you stay with the small or go up to the medium and large from there!
The Rockets are made of incredibly smooth PVC, are completely waterproof, safe for the body, and easy to clean and maintain. Wash them well in lots of warm, soapy water both before and after, or use a high-quality toy cleaner, foam, or spray. Any favorite water-based lubrication can be safely used with the Rockets; use a lot for an extra-slippery ride.
* Featured measurements list the thinnest Rockets Plug. The Medium is 4.25"/10.8cm insertable and 1.4"/3.6cm wide and the Large is 5"/12.7cm insertable and 1.9"/4.8cm wide