Right dimensions for pistures in slider

If you mean what is the slider area size, is 720×320 px.
If you mean what is the “visual” size, is 679×302 in 3 sidebar col. layout, and 599×266 px in 4 sidebar col. layout, as you have.

So, for a correct and optimized viewing in the slider, the photo that I must load must be 720×320 (as in the group photo I uploaded). Right?

No, not necessarily.

I did not understand the logic: if the image is too small, gray areas appear, as if too narrow. What are the requirements under that needs to have the photo to fill the space without being too big?

To fit the space must be 720x, but above you asked me for optimization, and image will be optimized even if smaller.
The difference is the empty area – not necessarily grey because you can color with the same background – but images are optimized anyway, i.e. the smaller images will not be blurry.
You just have to try.

Now I get it. Job well done, thank you.

Prego, marca il topic come risolto ??
You’re welcome, mark this topic as resolved

Giusto!