Start with responsive display ads
Teams often open by designing a full set of fixed banners. We would begin with responsive ads. Google builds the ad for each placement from the assets you give it, so you reach slots your fixed banners can't.
What to upload
Landscape image (1.91:1): 1200 x 628 recommended, 600 x 314 minimum. Square image (1:1): 1200 x 1200 recommended, 300 x 300 minimum. Square logo: 1200 x 1200 recommended, 128 x 128 minimum. Landscape logo (4:1): 1200 x 300 recommended, 512 x 128 minimum.
Keep the image clean
Keep text out of the image where you can, because Google adds your headline and description on top. Keep key elements away from the edges too, since images can be cropped to fit a placement.
Upload fixed banners only where you need control
An uploaded banner shows only on sites that have a slot of that exact size. When brand rules are strict, these are the five sizes Google's own data lists as best performing: 300 x 250, 336 x 280, 728 x 90, 300 x 600 and 320 x 100. The mobile one, 320 x 100, is easy to forget.
Design for the smallest size first
If your message fits 320 x 100, it will fit the larger ones. One idea, one logo, one clear action.
Check the file size before you upload
Uploaded banners max out at 150 KB. Responsive images allow 5,120 KB. Export them separately instead of reusing one file for both.
Add extra sizes last
Sizes like 320 x 50, 250 x 250, 120 x 600 and 468 x 60 are worth adding after your placement reports show where you are missing out.
More sizes do not mean more reach
Start responsive, add the five, and let the data show what is missing.