As a supporter, you will be wearing the jersey in informal settings - in the stands at the match, at the post-game barbeque, and in the pub. So it should fit you comfortably. You don't want a form-fitting Super-15 players jersey (even if you do have the physique to not look ridiculous). You want something that doesn't bind your arms, and that is comfortable in the chest, neck and shoulders. I'd suggest taking your tee-short size and adding 2 - see how that looks and feels. If you reckon you could be comfortable in it for 6-8 hours, buy it. Mark L.
choose the size that you would normally have for a T shirt or a dress shirt . i presume that is what you mean
