You will know this function already from the Frame editing module. When this icon is selected, you can only resize selected object frames proportionally. In practice this means that you cannot alter the proportions of objects – a circle will remain a circle and cannot become an ellipse. When the icon is selected the central grab handles of the object frame disappear, and only the corner ones remain available. But even their function is restricted, because only proportional size changes are possible. Grab a corner handle with the mouse (click and hold down the left button) and drag the frame out to the size required.