Any one of the large, rectangular banner sizes suggested by the IAB.
Specifically, the new rectangle ad sizes include:
300 x 250 IMU (Medium Rectangle)
250 x 250 IMU (Square Pop-up)
240 x 400 IMU (Vertical Rectangle)
336 x 280 IMU (Large Rectangle)
180 x 150 IMU (Rectangle)
Technically, all banner ads and button ads are rectangular. None are circular in nature. The word “rectangle” just happens to be the name given to these five new ad sizes.
see also: Digital Marketing Tools
Bigger Is Better
ClickZ (February 27, 2003)