this is not exactly what you asked, but just in case it helps you.
What i've been doing is adding a separator, not collapsable, no title. i've even modified CSS to remove its margins.
Thats stretches it down for me - but i'm only concerned with internal use, known screen res. so it works for us.
By using the ID, I'm sure someone can find a way to set the size of it or the form, but this may help for now if you didn't already do this.
jaymer...