I read other reviews and chose XL. XL is what a "real" XL should be. That's more XL than many other brands or clothing items. I'd say if you're usually between L and XL and want to wear it under a t-shirt or as an underlayer, go for L. XL makes a great top shirt to wear other clothes underneath. Now to the shirt itself. This is exactly what I was looking for: thin, stretchy base layer. It keeps its shape and is very light.
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