Here's how to apply a width or height value relative to the viewport:
Select the relevant frame or element
Go to the layout section in the design panel on the right
Expand the 'advanced' section
Set a value of your choice (in percentages) in the relevant fields (min/max width/height).
You can either type "vw"/"vh" for viewport width/height, or select these units by clicking on the tag unit in each input field.