To give the canvas more space to scale on different sizes like 600 x 800. I have it at 550 x 800 with the setting match 80% of the height.
You can program the camera to adjust itself when necessary. Test the screen width and height and if it's too far from your target, it can change the projection size (orthographic) etc. Just do a search on setting the camera size in Unity. There are multiple options.
Wouldn't it be nice if mobile screens all had the same resolution?!