If you're just beginning to explore the potential pleasures of butt play, start with the smallest jewel-based Spades plug. Feel free to move up to the medium and large plugs from there, or stick with the little one, it's completely your call!
Classic in shape, the sleeker tip of each Spades plug swells gradually outward to a largest point near the bottom. A nice long neck helps hold the plug in place as the softly pointed base circular base fits comfortably between the cheeks. That base also helps protect against any too-deep slips- a definite plus when it comes to anal penetration.
In pure silicone, the Spades plugs are completely hypoallergenic, non-reactive and safe for sensitive skin. All three are easy to clean using warm soapy water or a good toy care fluid/foam. The plugs are fully compatible with any favourite water-based lube, but please avoid contact with silicone lubes and other silicone toys/products. 
The Small is approximately 2.5" (6.35cm) insertable and 1" (2.5cm) at widest, the Medium is about 2.75" (7cm) insertable and 1.4" (3.5cm) at widest.