Always refer to the Size & Fit Guide offered on the product page of the item you're interested in. Take a look through other fans' reviews on that page, too, as there's a lot of great feedback about how it fits for others.
For any items you're unsure about, please measure according to the Size & Fit Guide.
Generally, we recommend you size up if you want your gear baggier and longer and down if you want it more form-fitting.