Get the document height and width

// Full height, including the scroll part
const fullHeight = Math.max(
    document.body.scrollHeight,
    document.documentElement.scrollHeight,
    document.body.offsetHeight,
    document.documentElement.offsetHeight,
    document.body.clientHeight,
    document.documentElement.clientHeight
);

// Full width, including the scroll part
const fullWidth = Math.max(
    document.body.scrollWidth,
    document.documentElement.scrollWidth,
    document.body.offsetWidth,
    document.documentElement.offsetWidth,
    document.body.clientWidth,
    document.documentElement.clientWidth
);