Ads for exchanges and linking sites should act as Mouse Click Magnets. Effective ads should draw your attention to them and get people to click. For shorter loading time it is suggested to keep it under 12k! So here are some criteria for making these types of ads:

• Under 12k
• Text messages need to be very short and legible.

Should be flashy and fast, or informative, but for 12 k may have to sacrifice one.

• Should conform to standard banner ad sizes.

As such there are no standardized banner sizes. Every website has its own size if its not just using Google ads. Still on the whole few sites prefer to use the following sizes for web advertising that are considered standard:
468 x 60 pixels (Full Horizontal banner)
234 x 60 pixel (Half banner)
120 x 60 pixel (Quarter banner)
120 x 120 pixel (Half banner)
125 x 240 pixel (Shirt column)
125 x 400 pixel (Medium column)
125 x 600 pixel (Long column / skyscraper)
88 x 31 pixel (Button)
88 x 62 pixel (Button)