I usually can fit in a size small in regular clothing but could get an AM in GK. I have a bunch of AS, but the most recent long sleeve ones have been mediums. AF has been known to be cut a little more generously, but it's hard to say, and I've never had a long sleeve style. The most important measurements is girth, then I'd go with chest, then hips. Use that priority to determine size. Don't go any smaller than the girth measurement, for sure, because the leo will not fit. But realistically don't go up like 3 sizes for chest measurement. My measurements are pretty true to the ratios they have, so I don't have much personal experience, but I'd say if you're getting AM in girth and something bigger in chest but you're average size, go no larger than AL. You may want a size up from the sleeveless fit because construction will be different, but use your discretion.