I want to test the appearance of a window on a smaller monitor than the one I'm using on the development machine. 
I tried with set_geometry_hints() (setting minimum and maximum width and height), set_resizable(False), set_default_size(), and set_size_request(). However every time the window is bigger, because child widgets request a bigger size.
I noticed on a smaller monitor with a resolution smaller than the request size, the widgets are truncated. I have to be sure this doesn't happen refactoring the GUI layout, so I want to simulate on my monitor.
How can I make the window smaller without truncating widgets?
Setting the window size request should be sufficient. If your UI makes the window larger that is the same as your widgets becoming truncated on a smaller monitor.
To prevent this you'll need to put widgets that grows your UI inside scrollable windows. Watch out for labels. You will need to get them to wrap properly.
Gtk3: set a fixed window size (smaller than the requested size from the child widgets)