I don't think you can mess with the borders or padding without affecting the widget's default size. You can set min-width
though.
From the docs: If this property is not specified, the minimum width is derived based on the widget's contents and the style.
QPushButton {
border-radius: 2px;
padding: 0.2em 0.2em 0.3em 0.2em;
border: 1px solid rgb(100, 100, 100);
background: qlineargradient(x1:0, y1:0, x2:0, y2:1,
stop:0 #f4f4f4, stop:0.1 #8F8F8F, stop:1 #7B7B7B);
color: white;
min-width: 70px;
}