There are several generally accepted sizes for ad banners.
full size - 468 pixels (6.5 inches) wide, and 60 pixels (.7 inches) high.
Half-size - 234x60.
400X40 - used by some banner exchange programs.
88X31 - used for link exchanges and the like. I like these much better than the larger banners as they load more quickly.
Some tips for banner creation.
There are many tools available which you can use to create banners. Adobe Photoshop is an excellent, though expensive, product. Paint Shop Pro is almost as full featured and less expensive. For animated GIFs you can use GIF Animator or Animagic GIF Animator.
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Unless otherwise noted, all photos and text is Copyright © Richard G Lowe, Jr.