how to set (preferred) JSlider length?

Hello, I want to set a length of a horizontal JSlider myself, and
then let the layout manager figure out lay everything out keeping my
length if it can figure out how. Some people say I should not set
sizes myself, but in this situation, letting the manager decide, I
think this is OK?

Anyway, I am not succeeding at all - here is my code boiled down to
the bare minimum - here I am not even using any layout manager. Here
is the JApplet init() method:

public void init() {
        JSlider s = new JSlider(0, 400);
        add(s);
        validate();
        Dimension d = s.getSize();
        d.width = 400;
        s.setSize(d);
        validate();
}

I try to do it twice, first in the constructor, then I validate() to
make sure it is really laid out initially, so I can call getSize and
get a meaningful result and then change it and call setSize.

My applet window size is 1000x1000. In the example above, I try first
without a layout manager. Then the slider size is 1000 as well, in
the middle of the applet.

OK - If I try, for example, to use BorderLayout and then first add to
Center, another component, and then this slider to the East - that I
thought would make the center as large as possible (this is how
BorderLayout works) and restrict the slider to the smallest size
possible, so now I hoped the slider would be 400 pixels wide and the
central component 600. No - the center is about 800 and the slider -
200.

What is going on here? Why can't I do this? How??

Please explain to me. Thank you very much! Mark