It is, yes. If you’re buying a cock cage to wear for more than just playing around, but particularly for overnight wear and longer, you want it as small as you can fit into as otherwise your nocturnal erections get going but stop halfway and that’s painful (really, just OUCH). If the cage is a snug fit they never get the chance ‘to take off’.
Unless you’d class your cock as above average or you’re completely a show-er and not a grower then start small and work up. The great thing about getting a CB6000S from Lovehoney is they offer no questions asked 365 day returns, so try a smaller one and if you really can’t fit, upgrade it, you’ll get it for free!
And if you’re skeptical about it fitting, just gasp at this:
https://chastityfrench.tumblr.com/post/160130406397/never-say-its-not-possible-it-is-and-its