How about
button1->setFixedSize(100,100);
button1->setIconSize(QSize(100, 100));
If your button lays on the toolbar then use
toolBar->setIconSize(QSize(100, 100));
instead of button icon size changing.
If you want to have different sizes on the toolbar then vary them with setFixedSize()
. Of course the maximal of them should be QToolBar icon size.