Just to expand on what yogeshgokul mentioned and if this helps...I believe you can also do
mainWindow->setMaximumSize(200, 200);
mainWindow->setMaximumSize(200, 200);
To copy to clipboard, switch view to plain text mode
and setMinimumSize() as well if needed depending on if you want the user to resize or minimize the window up to a certain point.
If you are using QT Creator and a .ui you can easily access these and more by right clicking the window in the designer mode and going to the "Size Constraints" option and galore you have 6 options I believe.
Hope this helps I'm kinda new hehe!
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