What is the proper header sizes for Jotform?

Hello,

I am having issues with uploading a banner image to the JOT form.  What is the appropriate dimensions for this?

If you are referring to the full size or width, the dimension is based on the width of your form. 

Click the Design button > Styles tab to know the width of the form:

To add the banner, use the image field from the basic fields section: