When working with the "Main Image" field (Plan > Configure > Display > Extended Display), the system defaults the form width to 800 pixels.  If you upload a Banner image larger than that, the page width will expand to encompass the banner. You can expand the width up to 2500 pixels and the height up to 300 pixels.

There is no "Recommended" Image size.  The system is flexible enough to set up your event and form however you want. You can go the basic route and just upload a header Banner image in the:

• Plan > Configure > Display > navigate to Assets
• Event Assets > click the Upload button > select the banner on your device
• Navigate to Extended Display
• Images and Attributes > select the uploaded banner on the Main Image  
• Click Save

Or you can customize the form even more by adding your own custom CSS and HTML, as well as uploading other images and resources in:

• Plan > Configure > Display > Extended Display
• Scroll down to Custom HTML Shell