You should be able to get a round button by using a stylesheet.
Add a QPushButton
in Qt Desgner and set its width and height to 40. Then right-click the button, select "Change stylesheet...", and paste in the following stylesheet:
QPushButton {
color: #333;
border: 2px solid #555;
border-radius: 20px;
border-style: outset;
background: qradialgradient(
cx: 0.3, cy: -0.4, fx: 0.3, fy: -0.4,
radius: 1.35, stop: 0 #fff, stop: 1 #888
);
padding: 5px;
}
QPushButton:hover {
background: qradialgradient(
cx: 0.3, cy: -0.4, fx: 0.3, fy: -0.4,
radius: 1.35, stop: 0 #fff, stop: 1 #bbb
);
}
QPushButton:pressed {
border-style: inset;
background: qradialgradient(
cx: 0.4, cy: -0.1, fx: 0.4, fy: -0.1,
radius: 1.35, stop: 0 #fff, stop: 1 #ddd
);
}
For more about styling buttons, see Customizing a QPushButton Using the Box Model and also the Qt Stylesheets Reference.