What is the best size image to use for the page background?

Hi! I am having issues setting up the page background image. What image size do you recommend so it fits?

I would suggest you to take a look on this link that will help you to know the size an image should have in order to be properly displayed in the form as background: http://www.webmalama.com/the-best-full-screen-background-image-sizes-for-web-design/ 

I have checked the image you're currently using and I can see it is a bit huge, it currently has 5175 x 2275.

If you would like the image to display in a better way as it currently displays then you could simply inject this CSS code to your form: 

.supernova {

    background-size: cover;

    background-repeat: no-repeat;

    background-attachment: fixed;

}

This guide will help you to inject it to your form: 

How-to-Inject-Custom-CSS-Codes

Here's my cloned form of yours where you can see how the code works: https://form.jotform.com/63065619159967 

Hope this helps. 

Thank you! That is brilliant!