Issue when i open window from mainwindow after the 1st time - it will open very small

[andreaQt]

Now this program do what i want, open an edit window, populate it like i want and i can update the mainwindow from the edit window.
I have some question:
1 - why when i try to do this setFixedSize( .... ) on a window (mainwindow or editwindow), the window that refer this setup have an issue: i can't close it by click with the mouse on X red button?
i avoid this issue by resize( .... ) + setMinimumSize( .... ) + setMaximumSize( ... ) ; i'm working on -> Qt Creator 4.11.0 Based on Qt 5.14.0
2 - i'm a beginner on Qt, an in progamming, is it the right way to structure the job?