It is important to know the best ad size for your website as it is an important way to increase your CTR and optimize your revenue. Along with knowing what the best ad sizes are, it is also essential to be aware of the optimal placement of those ads on your website. One must also think about what is not good to use as users tend to ignore older ad sizes which will lead to lower CTR. However, there is not one answer that works for every webpage but there are some general helpful suggestions.
Certain ad sizes do perform better such as the 300x250, 728x90 and 160x600 sizes. To an extent, some have been standardized and are called Standard IAB Sizes which the 300x250, 728x90 and 160x600 all fall under. Newer and less common ones tend to perform better and have higher click-throughs. The inline rectangle size (300x250) or the wide skyscraper size 160x600 performs well with the 300x250 size performing especially well. The inline rectangle size is a larger size and larger sizes do tend to perform better as well as wider formats since they are more reader friendly. The wider formats tend to perform better than the taller formats because they allow users to take in information in "thought units" through several words in a glance. Wider sizes allow users to comfortably absorb more words and not have to skip a line every few words and go to the next line while reading as you would have to with more narrow ads. Even though certain ads do perform better, it's important to take into account which ad sizes suit your webpage.
When considering what size of ad is best for you, it is also important to consider placement. If the ad's in a terrible place, the right size isn't going to do much. One optimal placement is placing an ad above the fold, near the top of the webpage because the ads that appear first on the webpage receive the most revenue per click. Also, ads placed near content rich areas tend to perform well. That being said, each size does usually has a certain spot on the webpage, for example, leaderboards (728x90) are likely to be placed close to the top of the webpage (above the fold) and skyscrapers are often on the right hand side close to the top or middle of the page.
There are certain ad sizes that get ignored more than others. Users tend to be used to older ones, automatically ignore and not even see them. Webmasters and those that have been using the internet for years are essentially ad-blind; they are so used to certain sizes of ads that they don't even see them anymore which is why non-standard sizes are optimal. Banner sizes such as 468x80 has existed since the internet was born, even people who barely use the internet can be blind to these ads.
There may not be a single solution in terms of optimal ad sizes and placement but there are several suggestions which can increase your CTR and ad revenue. The sizes 300x250, 728x90 and 160x600 tend to be higher performing, especially when given optimal placements. Placement is an important partner to the size of ad in terms of getting higher click throughs. Along with knowing what ad sizes are best, one must know what ad sizes perform the worst and tend to be ignored by users.