Well there are mainly two properties in which you have to look.
1. Auto Center
2. Auto Resize.
Once you fix the size of form. Auto Resize will make sure the form always opens to that size rather than how it was last saved. But remember that it is the edge of the background, not the window size which controls the final size of the form.
Secondly if you are using tabbed window rather than overlapping, they all gets the size of the access container. But if you are using overlapping, then the next property is AutoResize which should be True.