To reproduce:
Use the approach here:  http://docs.telerik.com/devtools/winforms/dock/object-model/customizing-floating-windows

Workaround:
private void RadDock1_FloatingWindowCreated(object sender, Telerik.WinControls.UI.Docking.FloatingWindowEventArgs e)
{
    e.Window = new MyWindow(radDock1);
    e.Window.FormBorderStyle = System.Windows.Forms.FormBorderStyle.Sizable;

}

class MyWindow : FloatingWindow
{
    public MyWindow(RadDock dock): base(dock)
    {

    }
    protected override CreateParams CreateParams
    {
        get
        {
            CreateParams cp = base.CreateParams;
            cp.Style |= NativeMethods.WS_THICKFRAME;
            return cp;
        }
    }
}