Thanks. My arms are quite skinny around the biceps, so I need to choose carefully I think.
size can vary alot ime across brands, I go between small and large sometimes with remarkably similar fits on the arm, obviously some end up ridiculously oversized but they tend to be one off brands that Ive bought because they were supposed to match a specific jersey and sold as a bundle, they always say buy based on jersey size, which is nonsense imo, because jersey sizes are predominantly focussed on body size, not arm size.
but Ive found Castelli/Sportful small or mediums a good fit on skinny arms, and uniquely still sell them, much like overshoes, arm warmers seem to disappear from some brands for periods of time or just never get restocked. I guess the price puts people off, but Ive always found them more useful than a gilet.