If you are under windows, then you can remove the Qt::MSWindowsFixedSizeDialogHint from the constructor which is set by default.
(I am not 100% this actually limits the dialog size or is it only a style used usually for fixed sized dialogs - but you can try it)
I don't have a way of testing this at the moment but my guess is that the QMessageBox is setting its size policy to fixed by default.
Try setting the size policy to something else and then set a new geometry/size.
I want it to be wider and have a better look (less word wrap).
What you mean by "less"?
It will either word wrap or it wont - how can it be less or more?
You might want to have a look at QMessageBox::setTextFormat()