Relative to the parent
Width and height utilities are generated from the \$sizes Sass map in \_variables.scss. Includes support for 25%, 50%, 75%, 100%, andauto by default. Modify those values as you need to generate different utilities here.
You can also use max-width: 100%; and max-height: 100%; utilities as needed.
Relative to the Viewport
You can also use utilities to set the width and height relative to the viewport.