This is according to the Q3 2003 ad serving trend report from New York-based DoubleClick based on data 1.1 trillion ads globally.
Standard banners (468 x 60 pixels) still account for the largest portion of all ads served at 58 percent, but declined in volume by 4 percent since this time last year. Buttons (88 x 31 pixels) lost ground over the past year along with the 120 x 90 pixel format. Skyscrapers, on the other hand, which were one of the fastest growing ad sizes, have started to taper off as a percent of all ads served, from 9 percent of total volume in the third quarter of 2002 to 6.3 percent last quarter. The square (250 x 250 pixels), most often used pop-ups or pop-unders, was used 33 percent less than a year ago, although it boasted the highest clickthrough rate this quarter, at 0.87 percent. Leaderboards, a 728 x 90 pixel-wide unit, became the third most common size, accounting for 8.8 percent of total ads served.
Sixty-eight percent of all ads served were IAB standard sizes; this number is relatively constant and fluctuated between 68% and 70% each quarter since Q1 2002.
Read the whole report here: DoubleClick Q3 2003 Ad Serving Trends.