I have a latex catsuit that when zipped all the way is tight around my neck. With collars my understanding is if I can get two fingers in between the collar that isn't restricting blood or oxygen. With the latex i'm not sure because it stretches. So even though I can fit two fingers between my neck and latex I don't know if thats an accurate way of checking with latex.
Do you have trouble breathing or start to feel lightheaded if you wear it for a bit? If not, then it is likely fine. If either are issues, it is too tight. I don't know of any other ways to tell to be honest.