Size of the background image recommended of the forms

Which es the size of the background image recommended of the forms? tsk

This will depend on the device where you will be loading your forms, for example, the size can be different if you only want your form to be loaded on mobile. 

We do not have a recommended background size, but the best practices for web image backgrounds can be implemented, here are some links that may help you with this: 

https://www.webmalama.com/the-best-full-screen-background-image-sizes-for-web-design/ 

https://graphicdesign.stackexchange.com/questions/41834/best-practices-for-full-width-browser-images 

Thanks.