There is a way to set the dimensions of an Icon in of a ToolBarItem in Xamarin.Forms?
It looks like that changing the size of the Icon only reduces the quality, leaving the Icon of the same size.
Is it needed a Custom Renderer? How should I do it?
You need to provide icons for the different screen sizes, for android the icons need to be in Resources folder in the platform specific project,
To explain Android uses separate subfolders using a naming convention based on resource qualifier tags. For example, the /drawables folder contains default image resources, while the /drawables-hdpi folder contains HDPI variants. Android will first search the current device density folder and then fallback to others if it cannot find a resource in the most ideal location.
You can find further details here