Most beginner photographers will immediately jump for the biggest box they can afford, but that isn’t always in your best interest. Shouldn’t a larger softbox cover a larger area than a smaller softbox? The reality is, softboxes cover about the same area, regardless of size. So what is the purpose of different sizes?
The quality of light that manifests in that area. Light quality is what communicates emotions. A small softbox will give you a harder light that implies strength & intimidation, whereas a softer light from a larger softbox will imply inclusivity & friendliness, and you will see more shadow building with a smaller softbox than you will with a larger softbox.
We’ve learned that that the area of coverage is very similar with various sized softboxes, but the quality of light is why you choose them. With that knowledge in your head, go out and have fun!