- Use a single image and Studio will resize automatically
- Upload the images to Asset Library to generate the different creative sizes
- Create one feed per ad size
- Create one feed and filter by creative dimensions
The correct answer is: Create one feed and filter by creative dimensions
Explanation: Filtering by the size of the creative allows you to select the right content for a specific size using a single column.
For example, you may want to use a short headline for a 160 x 600 placement, and a long headline for a 728 x 90 placement. Instead of creating one column for each (“Headline_160x600” and “Headline_728x90”), create a “Dimensions” column and a “Headline” column, then use the “Dimensions” column to filter your feed and select the correct headline.
Read more here: https://support.google.com/richmedia/answer/6023632