The internal diameter of the Beginner C-Sling is about 2" (note that the product specifications refer to the outer diameter of the ring). As most males are less than 2" in diameter, most will be unable to use this as a cock-only ring. Instead, the design of the C-Sling is aimed at use as a cock-and-balls ring, where one's testicles are inserted through the ring, followed by one's penis. Once everything has been worked through the ring, the balls are pushed forward, lifting one's cock up slightly, and enhancing one's bulge if you are wearing the ring under clothing.
Though the C-Sling is large enough to easily insert all your junk, the firmness of the silicone requires that you be soft to get in. Having any degree of erection will make it difficult to get the ring on all the way. Once on, the C-Sling is quite comfortable. The tongue puts a nice pressure on the perineum, and the edges are well-rounded to make them comfortable against the skin. The large size of the C-Sling may make it uncomfortable to sit with, unless you keep your legs spread. This may be a consideration if you plan to wear the C-Sling out.