Google display ad sizes
The single highest-inventory size.
Cropped in your browser · your image is never uploaded
Five sizes cover the overwhelming majority of display inventory: 300 × 250, 728 × 90, 160 × 600, 320 × 50 and 300 × 600. All are capped at 150 KB.
How to use the google display ad sizes
Google supports a long list of sizes but the distribution is heavily skewed, and building all of them is a poor use of time. The medium rectangle at 300 × 250 alone takes a large share because it fits both desktop sidebars and in-article mobile slots. If you build only two, build that and the 320 × 50 mobile banner, since mobile is where most impressions are. The 150 KB cap is the other practical constraint; it rules out photographic backgrounds at these dimensions and pushes designs toward flat colour and type, which is why display advertising looks the way it does.
Questions
300 × 250 and 320 × 50. Between them they cover most inventory across desktop and mobile.
150 KB for image ads.
It fits desktop sidebars and in-article mobile placements, so it is eligible for far more slots than any other size.
No. The tail sizes get very little inventory relative to the effort.
PNG for flat colour and text, which is most display creative. JPEG only if the ad is genuinely photographic.