This article explains the logic and lists the recommended image sizes for Howspace.
Howspace scales images as little as possible.
In practice this means that the images are scaled based on the width (either the container and widget used or the full width for backgrounds) and the image height is scaled accordingly to maintain the correct aspect ratio of the image.
It is recommended that the image size for backgrounds is at least 1920px in width.
To the widest container the recommended image width is 1140px.