All of the above answers are correct.
The theory is that both the fall off of light and the reduction in the effective size of the light source are governed by the Inverse Square Law.
What this means in practice is that every time you double the distance from light source to subject, two things happen:
1. Only a quarter of the light reaches the subject
2. The effective size of the light source is reduced to a quarter.
Let's go into a bit of detail here...
1. At a distance of 3' if the correct exposure is at f/11 then at a distance of 6' it will be f/5.6 (roughly). Not in itself a problem, use a higher ISO setting or turn the power up if you want to use f/11. BUT as
@Canon Bob says, the fall off of light will be different, at a distance of 3' the light fall off between say the nose and the ears will be much greater than at 6', simply because although the distance between these two parts of the subject are a constant, changing the distance means that the ratio of change has varied - let's call that distance 1' - at a distance of 3' the light travels 3' to the nose and 4' to the ears, so a difference of 25% in travel distance, at a distance of 6' the light travels 6' to the nose and 7' to the ears, so a difference of 12.5% in travel distance.
2. if the softbox measures 100x100cm, and if we take a distance of 3' as a starting point, at a distance of 6' the effective size will be 25 x 25cm, so won't really act as a softbox at all - one of the major qualities of a softbox is that it is generally larger than the subject that it lights, so the light hits the subject not just from the front but from all sorts of other directions too, the so-called wraparound effect. Obviously, the larger the softbox and the closer it is to the subject, the further the light from the edges of the softbox have to travel, so the light quality on, say, the sides of the face is different to that on the front of the face, i.e. the lighting power is less on the sides, and is generally more pleasing. If the softbox isn't big enough, or is too far away, that effect is lost. As a general rule, if the distance from softbox to subject is greater than the diagonal measurement of the softbox, it can't produce "softbox lighting". That statement isn't subjective, it's basic physics.
That's the theory. There are practical considerations too of course. Space is one consideration, and don't forget ceiling height, because typically, a softbox is mounted high and if you don't have enough ceiling height then you won't be able to get the best results from it. And sometimes, a large softbox close to the subject simply gets in the way physically, leaving just two possible solutions - use an even larger one from a bit further away, or use a smaller one and use post processing to mimic the effect of a larger one.
In a perfect world, you'll have several softboxes of different sizes and different shapes, even in a large professional studio with lots of lighting power. In a small studio, this becomes even more important. But, with care, knowledge and time, you can to some extent replace equipment with knowledge.
Yes, you can mask a large softbox to make it smaller, if you have the space, but you will lose a lot of flash power doing so.
