image size modern sharepoint pages

Hi,

Is there any documentation on the image sizes requirements for the modern page. I am trying to have an image as banner in the full-width section and in the preview it looks ok but as soon as I save it it enlarges the picture and I loose the text around the border. 

I have looked at this article but for me it does not make sense. 

I have created an image 1920x200 and it looks like this

but when I look add it to a full section in SharePoint it does this

Is there a default size for an image for it be banner and retain its border? 

Any help would be appreciated.

Dhiran