I'm going to get out my shirt and test those sizes (paper mockups) and see.
I've paid for the patch run, but there's still the process of finalising the design, so I can still change the size.
My current patch is 4" and it feels right. There might be a scale issue for me, because I'm 6'4" and 17 stone, so what feels right for my costume might not be right for somebody 5'6" or what have you.