Like the text size setting on the phone itself.
Phone: tapping a text box opens the phone's own keyboard on screen, so you see how much room the page keeps. Computer: type with your own keyboard and nothing opens.
Signed in shows the site as someone with a free account sees it, with a sample account kept only in this browser.
Hold any bank or ID card against the screen and move the slider until the outline is exactly the card's width. Browser zoom should be 100%.
Simulated browser bars. Sizes and timing are close to recent Safari and Chrome but not exact, and they differ between versions. Check on real phones before launch.