If you want a large ad unit on top of your site or in the page footer than you will most likely go for the leaderboard. The leaderboard is a very popular ad design and adsense places 4 ad units in the leaderboard. The size of a leaderboard is 728 x 90 pixels. The leaderboard design converts well on most sites that use this format in the header

If you want to put banner type ads on your site you should pick the 468 x 60 banner with 2 ads in it. This type of banner can be used very effectively on top of content or below it. The half banner only shows 1 ad and can be used everywhere on your site. Its size is 234 x 60 pixels. However if you want to go for a more vertical layout of your ads the 120 x 240 pixels vertical banner may be the one you are looking for

For small ads, place the Button ad format on your website. This format is a 125 X 125 square and holds a single AdSense ad. This can be the perfect showcase piece for an ad, especially when formatted with a background color.

A great converting unit for side display of adsense ads is the skyscraper. The skyscraper unit comes i 2 diferent formats. The regular 120 x 600 pixels to show 3 or 4 ads and the wide skyscraper with a size of 160 x 600 holding 5 ads

Several rectangle formats are used for in content display. The first one of them is the 180 x 150 pixels small rectangle holding one ad. The medium formt holds 3 or 4 ads in a 300 x 250 pixels layout and the large rectangle ad unit is measuring 336 x 280 pixels for up to 4 ads. The second square rectangle measures 250 x 250 and can also hold video ads

If you use image-based Adsense ads, you can choose the Leader Board which you will find placed across the top of many websites. It measures 728 X 90. The Banner is a smaller ad that measure 468 X 60. The Sky Scraper and Wide Sky Scraper are also choices for use on the sides of a webpage. Or you can choose the Medium Rectangle format. All of these image ads hold a single image.

There are also various layouts for displaying Link Unit Adsense ads so that they will fit well into your website also. They can be stacked, lined up across the top or bottom and allow just as much variety as the text and image ads.