It looks as if I would need to modify TQtWSCustomListBox in lcl/interfaces/qt5/qtwsstdctrls.pp to do something similar for TListBox.
Sorry, my answer was a bit sloppy. I just searched the sources for setUniformItemSizes.
The qt56.pas binding library has the function mapped only for ListView:
QListView_setUniformItemSizes
It is used by TQtListWidget.
TQtWSCustomComboBox uses a TQtListWidget for its dropdown list.
TQtWSCustomListBox also uses a TQtListWidget. You should be able to improve it without any new function mappings. Yes.