Currently the size of an StGroup depends exclusively on the group's children - it should be possible to override this behaviour with fixed values in the CSS. https://bugzilla.gnome.org/show_bug.cgi?id=613907
Currently if we want a stylable fixed-layout container, we use either a Clutter.Group inside an St.Bin, or we abuse St.BoxLayout. https://bugzilla.gnome.org/show_bug.cgi?id=613907