Triangles, squares, rhombuses, ovals, octagons— they are all shapes. In Spanish, when we simply want to say that something is a certain shape we would use Ser and say:
La forma de la pantalla de la televisión es un rectángulo.
The shape of the television screen is a rectangle.
And if we say that the TV is “rectanglular”, as in “rectangle shaped” (adjective), we also use Ser:
La pantalla de la televisión es rectangular.
The television screen is rectangular.
It is also common to say:
La pantalla de la televisión es de forma rectangular.
The television screen is rectangle-shaped.
Simply put— if you use Ser for shapes, you’ll be safe.