Very good trousers, indeed they have a slight stretch and are indeed made of cotton, good quality.
Since there is no store in Athens and only in northern Greece, and you have to order without trying them on, to help I will say the following:
I measured with a tape measure around where I button the trousers and it measured 104 centimeters. According to the Portwest size chart, the size that corresponds to this measurement is approximately 40 inches (or 56 European).
That's what I ordered and it is slightly loose (needs a belt) but it doesn't fall off. Maybe I would have preferred 38 inches (54) for a more precise fit. However, it is very good and comfortable when kneeling. Overall, it fits me well with a belt, it doesn't sag.