The scale in which the file is located when you download it is indicated in its product page, at the "Product details" level ("Original file scale") :

If the information is not present, do not hesitate to contact us, we will communicate it to you as well as the resizing ratio of the model to the scale you use. 

Note that some slicers may automatically resize objects when you import them: identify it when it happens to be sure to apply the right resizing.