At the top of the Properties Panel, there is a small width and height input field. By default, this is set to "Auto", which means that the button will resize itself based on the text inside it. However, you can manually set the width and height here, and the button will stay at that size.
If you want to make the button resizable, simply click on the chain icon between the width and height input fields. This will unlock the aspect ratio, and now you can resize the button by dragging the edges.