You just need to expand the Properties panel by clicking on the double-headed arrow icon in the tab (right next to the word "Properties"). The first click will probably collapse the panel, and the second click should expand it to its fullest size.
Looks like I had chosen the "Rounded Rectangle" rather than the "Rectangle" shape and the rounded rectangle's roundness can only be changed using the yellow diamond grips and not from the properties panel. But your reply was helpful too. Because I noticed clicking on that double arrow icon changes the extent of the available options in the properties panel.