Search assist would advise that to make the sidebar full length on every page in WordPress, you can use the CSS property height: calc(100vh + 200px); for the sidebar, adjusting the value as needed for your specific layout. Additionally, ensure that the sidebar’s height is set to be relative to the content height for consistency across different pages.
My caveat is that that you may want to consider keeping Sidebars on phones to contents only in height, else the user may get multiple empty Screens, if sidebar is controlled only by the page contents height.