I typically wear a medium, or a large if the clothing runs small.  I have had to purchase a medium and a large and neither fit.  I am waiting on an x-large now.  Be mindful of the sizes.  Very cool hoodie though, it seems to be a decent quality and should be very warm.
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X-large came and even this feels like it could be bigger.  I think it may be the nature of the jacket, it has the restricting feel of a suit jacket because it is multiple layers.  My brother got it for Christmas and literally wears it everywhere.  He has been using it in lieu of a winter jacket; I cant see it being warm enough but he swears it is.