Camp you have to put your controls inside a container, then you can make the label go left and the icon go right.
you could use a HBoxContainer for this, in layout set the label to expand so it takes up the whole space, and put the icon next to it.
set your HBoxContainer to the max width you want.
the problem could also be that the label doesn't have a minimum size. so just set a minimum size and change wrapping so it doesn't resize.