Hi Robin,
You can position the background image with this CSS:
#generate-section-1.generate-sections-container {
background-position: center center;
}
However, I always find it easier to crop the image before uploading so background image position can be tricky to deal with sometimes.
As for the height, it’s determined by the top and bottom padding:
https://docs.generatepress.com/article/sections-overview/#layout
Let me know if this helps.