I have sold many larger size jackets (48-52) that did not have a size tag. I always give them a numeric size in the listing title derived from the measurements. To this day, knock on wood, I have not had a problem. In the body of my listing, I always stipulate that the "size is approximate - please see measurements". In other words, if you give a numeric size when a size tag is not present, make sure that you have a little disclaimer.
good to know. disclaimer aside, i think the question is - on those larger sizes - how do you derive a numeric size based off of measurements?
do you stick to actual + 2"?