How can I tell what size a man's suit jacket is to put it on Ebay? I know my partner's size and it's too big for him! How can I measure the size of the jacket? Where to I measure it from? Across the shoulders at the back? On the inside? Arm length? And trousers - inside leg, where do I measure from and across? Thanks in advance!
oldwoman's advice is sound. However, when I use this method, I take the intended wearer's chest size as being an inch, maybe a couple of inches smaller than the actual jacket measurement, to allow for movement.
You could cover yourself in your ebay description by saying that the size isn't marked, but that you've measured it in this way and that it measures x inches. That would be easy enough for most people with a bit of commonsense to understand.