What size to create hero graphics with fixed height?

Using Wix Studio Hopefully this will be relatively easy to answer - I’m simply not well-versed in this part of graphic design and how best to communicate it.

I have a hero image set to Stretch, within a section of Fixed Height. The client will be creating graphics in-house and need to know what size to create to.

In this case, the height is 550px from screen width 1791px and up, so it could be 1800x550 or 2350x550 or 4000x550. The next breakpoint, from 1385px to 1790px, is height 500px. Since this doesn’t translate to any standard aspect ratio, how have you or would you recommend sizing these graphics at creation to fit the section comfortably and responsively?

Thanks!