11mm seems pretty big for a Metellina. Are you sure about it? Anyways, i agree with you, its most probably M. merianae (but you can never be too sure).
Thank you very much for your help. How I said the length was around 11 mm. It could also be 1 or 2 mm less. It is difficult to say because I had no measuring tape with me. But I would say that she was a bit bigger than Metellina segmentata. So I will call her Metellina cf. merianae.