How to create an independent, non-scalable element on the site?

Question

css

There is a script that is connected to different sites, it adds an element fixed to the bottom of the screen to the body of the site. The problem is that some sites have a meta tag installed and some do not, and therefore the block is displayed correctly on some sites and not on others. How to make the block not scale, and be equal in width to the width of the screen, regardless of the scale of the site itself.