how wide are the devices?

I was browsing online and noticed something. The length of the devices varies, so there seems to be one for everyone. It’s not quite as good as custom-made, but it’s fine for a start, worth trying.

However, the diameter is always very large—35mm or more. For me, 30mm would suffice when soft. If I buy one with the perfect length, it ends up being too wide.

Is there some practical reason I’m missing? Is it that you can’t get much longer but need space to expand?

I have to settle for a “big” diameter device. Even the ones labeled as “small,” which wouldn’t fit me otherwise, are 35mm in diameter. And I never realized before now that my penis is unusually slim.

Usually yes, the advice is to go shorter and wider. You want room for growth to prevent constriction, and it also makes cleaning and moisturizing easier. As far as pre-made devices are concerned, they likely just make them extra wide to ensure a fit for most men, since there isn't really any downside to doing so. After all, I think everyone knows that off-the-shelf devices are really designed for short-term wear; they don't require—and it wouldn't be reasonable to expect—a perfect fit.