You can position and scale objects two ways in Google Web Designer – using pixels or using percentages. When you use pixels, you're specifying an exact placement of an element or its exact size. When you use percentages, you're specifying the position or size as a percentage of the page (or enclosing element). Percentage-based layouts are often helpful for responsive or fluid design.
Use percent-based values
To keep elements positioned relative to the ad regardless of viewport size, set layout-related attributes using percentages rather that pixels. For example, when an element is positioned at 20% of the page size, it will adjust to stay at 20% as the page size changes.
To choose between pixels and percentages:
Click the measurement unit (px or %) and select the unit that you want to use from the pop-up. Percentage values are based on the size of the enclosing container.