QMainWindow Fixed size problem

Hi, I am currently learning Qt and have written an application using QDialog which I set to be a fixed size, because I now want to include a menubar and toolbar I have changed to using QMainWindow but I now have the problem that the Application is resizable even though I have set the 'sizePolicy' to 'fixed' within Qt Designer as well as setting up the 'minimumSize' and 'maximumSize' properties to be the same. What am I doing wrong or not setting up properly? I am using Suse 9.1 and the tools that it comes with. Regards Kirk