IMU are the standard ad unit sizes endorsed by IAB.
Common 7 IMU’s are :
300×250 pixels – Medium Rectangle
250×250 pixels – Square Pop-up (Utilizes Java)
240×400 pixels – Vertical Rectangle
336×280 pixels – Large Rectangle
180×150 pixels – Rectangle
160×600 pixels – Wide Skyscraper
120×600 pixels – Skyscraper