What’s the size for small

I got the size "Large" for my husband who wears a 34" and they fit him to perfection. -- Now, their size charts say that a Medium is a 32"-34" and a Large is a 36"-38" Based on that information, I would tend to go up one size. Or, read the size chart, and wherever his waist number is, choose the next size up.

My waistline hovers around 31" which really is very frustrating to buy for. should i risk the sm or the med?

I have a 33 - 34 waist and purchased the Med. They fit perfect.

How are these bamboo fiber (Rayon) compared to the micro modal?

Just an FYI - no one, and I mean no one should be wearing Micro Modal or any fabric related to rayon or viscose. These materials are only semi-natural whereas the synthetic part uses chemicals in its creation process that are cancer causing and do not actually breathe but rather give the "feeling" they do. In small amounts, like underwear or socks containing 5-10% or less of this material for the stretching purpose, it can be usable. However, it's suggested that you air dry these items as to not release any chemical attributes in the washing process. When in doubt always stick to 100% cotton, bamboo or other natural material - especially when its that close to your manhood.

What is the difference in pouches? ergonomic, u, enhanced u, 3d. i'm confused.

The pouch is described as a 3D design and it puts things out there more so than the brief version (which is also quite comfortable). I find it comfortable for my male anatomy. I wouldn't say it enhances as much as say Andrew Christian or AussieBum underwear, but the material is very comfortable and I think that's what sets it apart from some of the other underwear I've bought.