According to the GNOME design guidelines all primary GTK windows should be resizable:
Note on the current state of things after bug 7902:
* Resizing is explicitly disabled "self.set_resizable(False)" in tlgtk/wizard.py
* Things "work" when resizing is enabled but do not adhere to responsive design principles. One example is that things just grow indefinitely when making the window larger.
GNOME's libadwaita includes some stuff that could do a lot of the heavy lifting for us (like "AdwClamp"). Unfortunately, it is quite new and is not available on older distros.
We could set the bar lower for this, the responsive design aspects could be handled as a different issue. In that case mostly what remains on this bug is testing.
When making changes for bug 7902 we have tried keeping this in mind, but since our goal for that bug didn't include resizing it has not been a priority.
Only shallow testing with "set_resizable(True)" has been done. While things didn't look great, we didn't encounter anything that strictly didn't work or become unusable.