The Medium Rectangle (300×250)
This ad format supports both display / text ads, and it can also be used on mobile layouts. The best part about this ad size is that it’s easy to fit in the sidebar or inside the content area without annoying users.
That’s why this ad format gets a larger ad inventory which means more and better paying ads for your site. Since it’s perfectly-sized for in-content ads, it is also reported to have the best CTR for most websites.
Large Rectangle (336×280)
Large rectangle is larger and more noticeable than the medium rectangle ad, but it is not mobile optimized.
That being said, the large rectangle could easily outperform medium rectangle and is sometimes placed inside the posted article or information page area between paragraphs. It supports both image and text ads.
Due to its size and ideal placement next to content, this ad size gets a better ad inventory as well.
The Leaderboard (728×90)
The leaderboard is a traditional banner ad size that is perfect for website header. It supports both text and image ad formats but doesn’t support mobile ads.
Since the leaderboard is ideal to be placed on top (inside or immediately after the website header), it gets more ads which means better ads for your website. Due to its size and prominent placement, it performs fairly well on most websites.
Half Page Skyscraper (300×600)
Large Skyscraper is a wide vertical banner also known as half page due to its size. It gets attention from advertisers who want more ad space to effectively communicate their message during retargeting.
It is becoming more popular among publishers because it tends to give more exposure for brands which means better paying ads.
You can place the large skyscraper in the sidebar or next to the content. However, due to its size it may not be a good fit for all websites.