To the Right of the Builder is the Properties Panel. This is dynamic depending on the screen/layer you have selected and will automatically change accordingly, bringing up settings for the current Tools/Components selected.
- Responsive Screen – if selected, it will auto-scale your ad unit (excluding Wallpapers) according to the browser dimensions of the user. If you have several screens, be sure to select this for each screen. * If the ad format is already a fully responsive format, this box will not appear as the format will automatically be responsive.
If you are looking to have multiple creatives (1 for Desktop, 1 for Tablet, 1 for Mobile), also see Delivery Group: Responsive Width