When defining ad sizes we use the number of pixels wide and high the creative is to describe the size. For example a 468x60 banner is 468 pixels wide and 60 pixels high. The ad sizes a website will use is dependent upon the layout of their site. It is encouraged to stick to the standard sizes so as to reduce the workload for ad agencies who may have to create a large number of creative sizes for the different publishers.
Some standard ad sizes, formats and specifications are as follows:
A large advertisement typically measuring 728x90 that appears at the very top of a web page.
A medium size advertisement typically measuring 468x60. Usually placed at the top of a web page but can also be placed within the content in the middle of a web page or at the very bottom of a page.
A medium square or rectangular advertisement that appears within the content of a webpage. Typically measuring 300x250 but can also be any of the following sizes: 200x200, 220x220, 225x225, and 250x250.
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A small square or rectangular advertisement that can measure anything from 12ox90 pixels high to 120x60, 100x100, 120x120 or 125x125. Buttons can be placed anywhere on a web page but typically appear down the left or right hand sides.
A medium size advertisement typically measuring 120x600 pixels high that has the silhouette of a high rise building. Usually placed down the left or right hand side of a webpage. Other sizes include a 120x300 mini skyscraper or a 160x600 wide skyscraper and a 300x600 half page skyscraper.
A medium sized advertisement that appears over the content of a web page and is served by a 1x1 placement but typically measures around 400x400 pixels high or less. Overlays must have a close button and should automatically close after 30 seconds so as not to interfere with the user’s experience.
An advertisement located in top left or right hand corner of a web page that appears to peel back like the corner of a page in a book or magazine. Upon a mouse over or click the ad expands up to half of the size of the screen or more displaying the advertiser’s message.
Teaser 
Expanded
An advertisement of any size that opens out in one or more directions displaying over the content of the web page. For example a 300x250 add expands to a 300x500 size. The expansion usually occurs in response to a user action such as a mouse over or click and should retract on mouse off or click.
Collapsed
Expanded
