The shirts aren't that hard to measure, so you are probably fine buying off of eBay unless the seller is a moron and can't measure. Anyways, my thoughts on the shirts is that while the westerns I've tried on aren't any slimmer than the regular workshirts. The difference to me is they taper more in the body, but nothing extreme. Compared to the selvedge oxfords, the oxfords slimmer all throughout the body and sleeves as well (at least the solid colored ones, white one maybe less so). Really the only thing to look out for is shirts that are meant to be overshirts, those are the only shirts I would be aware of as they fit larger for that reason.