The sizes you need to configure in DoubleClick for your ad units are:
- Bottom and Inline banners
- iPad: 768x66 (portrait)
- iPhone: 320x50
- Android: 320x50
- Android tablet: 800x90 (portrait)
- Splash banners
- There is no specific size. This is an HTML page, therefore you just need to make this HTML mobile responsive and it will be displayed correctly.
FYI: You will need to configure a adunit for each device, example: (iOS) iPhone, iPad & (Android) Smartphone, Tablet.
As for the file formats and more, please refer to the Supported DFP Advertising Formats guide documents here.