For every item you find under "Fit and size" on the product page how the item fits: true to size, slightly smaller or slightly roomier. That tells you whether to choose your usual size or go one size up or down.
Many of our items are "one size". That means they come in a single size that suits several clothing sizes thanks to the fit. Which sizes that covers is stated per item under "Fit and size", for example "suitable for sizes 36 to 42".
Still in doubt? You always have 14 days to change your mind: if it does not fit, you can exchange it for another size or return it.