It seems that you can't change the width of a spacer (because it's an "imaginary" object?).
But you can move the visible elements around with the layout editor - as long as the group width is <= 30
So the problem is the layout editor has a maximum group width of 30. Is this a bug?
Btw. This is also true for me. Both, Node Red and Dashboard most recent versions. While already existing spacers have different sizes (from older NR versions) the newly generated ones are restricted to 1x1.