To create different banner sizes, click the Banner Sizes tab in the upper right corner of the banner size.
To edit edit an existing banner size, select the size name you want to edit underneath the size name field.
To add a new size banner, click the green + Add a New Size button.
This will put you in the Add a New Size menu. Here you can enter a size name, the width (pixels) of the banner, and the height (pixels) of the banner.
When you are finished typing into each of the fields, click the Save Size button to finish, or click the Save & Add Another button to save your current size and add a new size. (You may also click cancel to cancel any changes.)