Rectangles and Pop-Ups

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)
300 x 100 IMU – (3:1 Rectangle)
720 x 300 IMU – (Pop-Under)

Banners and Buttons

468 x 60 IMU – (Full Banner)
234 x 60 IMU – (Half Banner)
88 x 31 IMU - 
(Micro Bar)
120 x 90 IMU - 
(Button 1)
120 x 60 IMU - 
(Button 2)
120 x 240 IMU - 
(Vertical Banner)
125 x 125 IMU - 
(Square Button)
728 x 90 IMU – (Leaderboard)

Skyscrapers

160 x 600 IMU - 
(Wide Skyscraper)
120 x 600 IMU - 
(Skyscraper)
300 x 600 IMU – (Half Page Ad)

Other

425×600

Sources: *