The YouTube Masthead is a customizable 970x250 pixel ad that runs on the desktop YouTube homepage for 24 hours. The Masthead can also expand on user click to 970x500 pixels. YouTube highly recommends the use of videos in custom Mastheads to provide the most compelling and native ad experience.
Process and specs
The Custom Masthead Guide in the Display Specs Help Center explains everything you need to know about the process of launching a YouTube Masthead.