My cat is not too big (4.5-5kg) so the M size is totally fine for him. If you have bigger cat go for the L size.
I like the bag a lot. It folds easily and I can use is in an car as well because M size is small enough to fit under passengers feet.
The best feature about this carrier is that it holds its shape when it's set up. Previous bag i had would have some metal spines inside it that would fall out and the bag would just crumble making it uncomfortable for the cat if it was inside the bag.