No need to be sorry, thanks for the input!

May have found it in a header file: NautilusListIconSize in nautilus/src/nautilus-enums.h. I might try to add a smaller icon size level

Edit: Or maybe I better play around with margins/paddings of rows/cells. They seem to be too large in my screenshot, comparing it with yours.

Dconf editor lets me define the default list view size, but does not allow to set smaller list view size than in my screenshot.

Thanks, but I try to avoid nemo at the moment, as I face a problem of not being able to paste images of a specific types into it. Else it is a good program!

No, the size in the screenshot is lowest size available. I use version 45.2.1, although only installed Nautilus, not the full GNOME shell.