This infographic provides a great look at the different sizes of display ads that Google Adwords offers.
Display Ads can appear in a variety of shapes and sizes depending on the available space on a page and its not always easy to know what shapes and sizes your ads may appear in when the general public sees them. This infographic gives you an easy to digest look at all of the different sizes that your ad may be adjusted to.
We would always recommend putting a text version of your ad live alongside your image ads on the display network. Not only does it insure you have an ad to cater to every dimension, but we have also seen these generate higher click through and conversion rates with some of our clients.