It appears you are using a SummaryDetail page in the modal window workspace.
I got the same result as you did. The SearchSummaryDetailPage seems to work OK, though, and I found that on construction, the dock style for that page is set to Fill.
Apparently, the WindowWorkspace doesn't set the dock style of the smart part it displays.
Try this:
In your page controller, override CreateViews():
protected override void CreateViews()
{
base.CreateViews();
((
Control)this.MainView).Dock = DockStyle.Fill;
}
It's a hack, but it gets the job done.
Bill J.