Canvas, instantiate and adjust size of image to fit parent

Hello everybody, Basically I want to instantiate an Image and make it fit in a panel.

I can instantiate my Image, set it as a child of a specific panel and then change its anchoredPosition to (0,0). Making mi image apear on screen on my desired position.

The problem is that when I increase the resolucion or hit the "maximize on Play" button, my image appears but doesnt change its original size, so it looks smaller .than the panel.

I need to know how to change its size to adjust to the dimesions of its parent.

thanks everybody