I don't have any around and I'm designing a custom case and was wondering if anyone had this dimension handy. I don't want to pull my computer apart to find out!
There is a standard, and that is that they come in different sizes
Depends on which kind you are referring to, most are about 5mm to 1/4 inch depending on manufacturer.
The screw in kind can be almost any length, I have seen some that are 1 inch at least. I'd plan on 1/4" for a custom build unless you want space behind