Be aware that at least on my browser, the itty bitty ruler icon that David talked about goes away once you start clicking on the other sizes. If that happens, just reload the page and don't touch the size icons. I'm using Safari (not exactly a rare browser at 13% or so market share) so I think it's just shoddy configuration on their part.
To loop back to my question, I measured a few shirts and they're all around 20" wide, buttoned, and that's equivalent to about 40" circumference/chest. I don't think I'm a particularly small fella (5'10"/160), so that's why I asked for a confirmation from those who actually have these shirts, especially as they are "Final Sale".