Of course, this is one of those questions that are insoluble to those who are not actually living your own life. But, there are some rules of thumb to go by that might help you to understand diffuser (softbox light is diffused light) light sources.
Most people, even some photographers, mistakenly think that the softness (lower overall contrast, with lighter shadow values compared to harder lighting having comparable highlight values) comes from the evenness of the light source, such as light that is bounced off a large reflecting board or diffused through a medium such as translucent cloth, vellum or plastic. This is only about 1/4 right. The "softness" from these light sources is actually solely due to the overall size of the light emitting surface in relation to its distance from the subject. If a source is very uneven, with much brighter areas surrounded by those less illuminated, then the size of the source can mainly be measured by the part of the source which is most bright, like the light coming from an old fashioned photo flood fixture, and would actually measure as smaller overall than the size of the reflector. This is why the most efficient way of delivering soft lighting from a source of a certain measured area will usually be from an evenly illuminating source such as a softbox, light reflecting off a board or wall, or even a well made umbrella. But, the real data that counts is still how big the illuminating source is compared with the distance to the subject.
Understanding this should give you an idea how to calculate how big your softboxes should be to work for your subjects and your preconception of the quality of the light. If you cannot vary the distance from multiple illuminating sources to your subject by very much, then, in order to change the soft-hard quotient of the lighting from more than one softbox source, one would require different sized softboxes to accomplish the desired effect. Also, things such as overall space requirements, weight on - and position of - the lighthead (torque strain), portability, cost and many other factors might lead you to choose boxes of different sizes.
Think about what you may have to shoot and figure it out for yourself.