The short answer
The banner sizes with the deepest advertiser demand — and therefore the best fill and CPMs — are the 300x250 medium rectangle, the 728x90 leaderboard, the 160x600 wide skyscraper, and the mobile 320x100 large banner and 320x50 banner. These are the IAB standard sizes buyers target most, so supporting them well is the baseline for competitive eCPM.
The sizes that matter
300x250 — Medium Rectangle
The single most valuable and versatile size. It works in-content, in sidebars, and on mobile, and it has the deepest pool of advertiser demand — including rich media and video. If you optimize for one size, it’s this one.
728x90 — Leaderboard
The classic top-of-page desktop banner. High visibility above the fold, strong demand, ideal as a header unit on wider screens.
160x600 — Wide Skyscraper
A tall sidebar unit that stays in view longer as users scroll, giving it good viewability on desktop content pages. (The 300x600 half-page is a higher-impact cousin with strong CPMs where the layout allows.)
320x100 and 320x50 — Mobile Banners
The mobile essentials. 320x50 is the standard mobile banner; 320x100 (large mobile banner) earns more where the layout has room. With most traffic now mobile, these carry a large share of impressions.
300x600 — Half-Page
A large, high-impact desktop unit with excellent viewability and strong CPMs — worth supporting where your layout can accommodate it.
Responsive and fluid units
Modern publishing rarely hard-codes a single size per slot. Responsive ad units let the ad server pick the best-fitting size from a set for the user’s screen, which improves fill and viewability across devices. The practical approach:
- Enable multiple sizes per slot so buyers can compete with whatever creative they have.
- Prioritize the high-demand sizes above within each responsive slot.
- Match size to placement — don’t force a 728x90 into a narrow mobile column.
Beyond size: what actually drives earnings
Size sets the demand pool, but revenue also depends on:
- Viewability — placement and sticky behavior matter as much as dimensions.
- Competition — running each slot through header bidding so multiple buyers bid.
- Page performance — more/larger units can hurt Core Web Vitals if not loaded cleanly.
The bottom line
Support the high-demand standard sizes — 300x250 above all, plus 728x90, 160x600/300x600, and the mobile 320x100/320x50 — and serve them through responsive units so buyers can always compete. But size is just the entry ticket: viewability, real competition per impression, and page speed decide what those slots actually earn. Fit them into a deliberate format mix and tune with yield optimization.