Percentage height and width
Elements are able to shrink or expand when their sizing is defined by percentage (%) instead of pixel (px). This means once you change an element’s height or width to percentage it will be responsive to the simulation window’s height or width or the container that the element is in.
To define the sizing by percentage, go to the Properties palette and click on the drop-down window next to the width or height, then change it from “px” to “%”. The number that shows will now refer to the percentage of the height and width instead of the size in pixels.
Note: For this to work, the Simulation settings must be set to ‘Expand to browser’s width’. To do this, you can go to File – Simulate – Settings and click on the drop down menu and select ‘Expand to browser’s width’.