Thinking of leather and NATO actually. I mean I don't know how to choose size. I'm noob in this. What should I look out for when choosing a strap online?
LLD is 22mm lug width.
My advice is try to avoid leather straps with curved ends at the lugs as they may not suit all case types. The flat end ones are safer.
Off-the-rack straps can also be a problem for someone with small wrists, when you are wearing a big watch or when the watch has obscene L2L distances. In which case, there exists a good chance that the smallest hole wouldn't fit snug on your wrist (that's just a preference).
Fortunately, there are bespoke leather straps services available to cater to all needs, with the downside being the higher prices commanded.
For natos, they're generally cheap and tend to fit most wrist sizes. If your wrists are smaller, you may want to avoid nato straps that are too long (like Maratacs for example) as they tend to have a long "tail" that you may not be able to fully tuck under.