Thanks everyone for your advices. I managed to find a shop that had similar bikes in both sizes (other version, but the same frame).

Reach in both seems to be ok, although I slightly prefer L. It gave me the feeling of having more space. Would a longer stem help with that?

But the dropper height was too high for me. When I put the dropper fully in frame and extend it, I was barely touching the pedals. In M/L with the same setting the height was in point.

So although I still have some doubts, I think I will buy M/L. For L I would definitely need to swap the dropper to shorter one. For M/L I may need to buy longer stem, maybe 70mm rather than original 50mm to get some more reach.

What is your current bike? If you are comfortable on it I’d be happy to help with a comparison against the M/L and L.

The last one that I used didn’t have any marks about size, I bought it just be trying it. One of my older bike that I remember was B’Twin Rockrider 520, size L which was good size. Bot completely different geometry and different usage. Here’s the geometry of it https://contents.mediadecathlon.com/p1969850/k$7dbe52fe42d6acefb052002ab55a3b28/geometria%20ST520%20530%20540%20jpg.jpg