You can ctrl+shift+click this to set the anchors and alignment automatically for the overlay that sits in the canvas.
Image & Button H/V alignement are set to Fill to they conform to whatever the text size there is.
No need to worry about the text itself, Size to Content of the canvas’s slot will take care of everything anyway.
But again, if the button or the image are larger than the text (because this can be overridden), then they will start dictating the size of the overlay.