Regarding size, L is quite capable of housing a 5-inch phone - a phone Quechua 5 without loss at the screen or at the frontage keys. By cons, in terms of the side keys, it's a little trickier, but we seem to find them.
Edit- after test - However, it is true that the folds of the transparent part does not facilitate the use of touch. To connect the phone to an external battery, it can also be tricky to test whether the rigidity of the case does not pose a problem to the passage of the cable.
Concerning the strength of the assembly, I confirm the robustness of the support, a very comfortable margin over the imposed constraints. In fact, this is the bag itself, which will absorb the bulk of vibrations, so that the constraints on the media are not as important as they seem. It gives confidence, and the telephone is readable (unless there is too much sun - ').
However, the L version supports some space on the handlebar. I will add a photo to mark the site.