If you have even tried to draw a circle or square on a slide, you might have noticed that the tool is actually called ellipse or rectangle which does not automatically give you a perfectly proportional shape.
The next time that you want an evenly proportioned shape, select the shape that you want in Insert tab.
Press and hold down [Shift] and then draw the object. Release the mouse before letting go of the [Shift] key and you will have a perfect circle (or square). Hold [Shift] again if you need to resize the shape.
This trick can also be used for drawing other shapes including stars, octagons, triangles, and more.