A speed square is used primarily during framing. As I show in the Tiny House Design & Construction Guide, you use it to mark your stud locations on both the top plate and kick plate at the same time. Because of this, it is sized to be right around 7 inches long, the width of two 2x4’s.
I prefer a speed square that is made of brighter aluminum like the Empire 7 in. Laser Etched Aluminum Rafter Square. It is lightweight, inexpensive, and its bright color makes it easy to find. The laser etching makes it easier to read, although I rarely ever actually need to read it since it is normally just used as a straight edge.