How to get real image size on viewport?

I created a widget (user interface) and put a image in the upper left corner (inside of the canvas).

I want to know how to get real image size on viewport. Because right now i’m getting the size like this.

And this way is not useful for my purpose

Because i want to convert the player coordinates on the world to coordinates on the image.
So i need the real image size.

How can i get it?

Thank you so much!!