Hi @Roxzfr, thanks for the good question, I think it depends on the layout, but if putting the hero image in a container, then 940px.
If creating a full browser width hero section, then a good thing to think about is what is the maximum target desktop view that you want to optimize for.
I generally use an image that is 16:9 aspect ratio and 1920x1080 or 1400x787 (for smaller views).
Here is an aspect ratio calculator: http://calculateaspectratio.com/
Here is a good blog post on working with images and performance: https://webflow.com/blog/how-to-boost-your-sites-performance
I hope this helps!