Another relative size unit, not yet discussed is viewport units. A viewport unit is either defined as vh, for height ratios, or vw for width ratios.
1vh is defined as 1/100th of the height of the viewport or browser. This means that 100vh is the full height of the viewport window.
This can be particularly useful when working boxes that you want to be or maintain a height that is in relation to the height of your browser.
Likewise, 1vw is 1/100th of the viewports width. However, you must be careful when working with vw units, as they are based on the width excluding scroll bars. On macOS this is not a problem, but on Windows, 100vw will not fit in a browser, as the scroll bars will cause it to need to reflow. Therefore, you may be bast served to use percentages when setting width.
For more on viewport units, please read;