How can I change the font size of a GWT Label object dynamically?

In a GWT project, I want to be able to do something like this, to set the font size that'll be used for an instance of a Label:

public class BigLabel extends Label {
    public BigLabel(String s, int i) {
      super(s);
      setFontSize(i); // sets font size in em units
    }
}

But I can't because I can't find any method that does the same job. I can easily use Label.setStyleName, 开发者_如何学Pythonbut if I do that I have to set it to an absolute value stored in the css file. Is there a way to have a font size that's flexible?

private void setFontSize(double ems) {
  getElement().getStyle().setFontSize(ems, Unit.EM);
}