How do I find out screen size?

I'm trying to make a (simple) mobile game that works on multiple screen sizes and aspect ratios. My idea was to create a large layout and then have a relatively small viewport. I can then use a scale outer to run fullscreen and just set an invisible bounding block to the edges of the screen.

I've tried:

set width = viewportright(0)-viewportleft(0)

set height = viewportbottom(0)-viewporttop(0)

construct.net/en/forum/construct-2/how-do-i-18/size-different-screen-108642

But I don't know if it's working. My width always seems to change whether I change the width OR height of my browser window. Likewise my height never changes.