I'm a US-Large so I ordered their 2x which is 2 sizes up and its still a tad small and the sleeves a tad short. Its not terribly small and I can wear it but it would be better if it were another half size bigger. I also happen to like my long sleeves below the wrist but if this isn't something you care about then this will fit you ok. However  If you're a US-XL then this will be too small in every way.  Being "slim and fit" won't save you. You simply can't wear this. I usually stick to short sleeve shirts from Asia as they have a much better chance of fitting but you still have to go up a good 2 sizes always. Some Asian stores do free custom alterations before shipping and obviously that's the best way to go.