Bolt sizing: how can you tell the size (M4, M5 etc)?

I want to replace some rusty fasteners on my SS with new stainless socket head allen bolts. Need to know the size to order though.

Do you just measure the widest part of the thread on the bolt? M5 = 5mm?? Or am I missing something?

Nope you are pretty much spot on there.

As for the length, you don’t count the head of the bolt, just the threaded or partially threaded/straight bit depending how long the bolt is. I hope I made that clear enough.