Personally, I'd prefer at least one bigger one (about 1.8m or more) for dance. You can of course make the spread of light bigger from any light by backing the light off. This changes a couple of other things though which you may not want: the light becomes smaller, and so harder (less wrapping, sharper shadows); overall intensity of the light at the subject will now be less, as the same amount of light has now spread out over a larger area. It also means the rate of fall off across your subject will be less - ie the light will be more even. Now, of course you can solve the first problem by turning the light up, but that then means the duration will be longer, so you may opt to increase the ISO or open up the aperture (and these things also have side effects
like increasing the brightness of other light sources in the room, and (for ISO) increased noise and less dynamic range). It all depends on what is most important to you for the image, and how much wiggle room you have on each component. eg if you increase the light power and duration - does it actually cause a problem? Is the subject still sharp or do you see movement blur? Is the blur a good thing or not? If you don't want any blur, it's usually best to opt for a bit more ISO instead and keep the light power (and flash duration) low, check the room lights still don't show up (shoot without flash) and accept a bit of noise - tbh on modern cameras a bit more gain (ISO) is perfectly fine.
The biggest problem with backing the light up in my experience though is that it's going to bounce off walls, and ceilings more - and of course you turned it up, or raised the ISO etc so the weaker bounce light now shows up more too - ie you reduce the contrast between your lit subject, and the background. Of course if you want an all-white background and have already thrown lots of light on it then a bit more won't be a problem - but you will still get more light bouncing back onto your subject, destroying shadows etc.. You could turn the background lights down, and this may make the background uneven, but I know that's easy to correct that in post... and so on.. (this is where your brain may start hurting a bit as you try to resolve all the side effects on the fly, whilst not stalling, with your head in your hands and still looking like you are in control and instilling confidence in your subject!
- the ultimate answer is to have a gigantic studio
)
Here's a small 140cm strip box lighting a figure in a small room with white walls. then a 1.8m one roughly half the distance away and turned down. See how much darker the room is? In fact at 1.8m the light is physically bigger than the subject, and will still have enough spread to light her from head to toe even if we put it inches away as in the 3rd image :-
View attachment 118229 View attachment 118230 View attachment 118231
In most of my dance shots, the lights are right on the subject (I just remove them in post) You can also add grids to help contain the light, but then they reduce spread ! Here's the big one with a grid fitted, and power turned up to compensate
View attachment 118232
Of course you can also use flags - black sheets of something - cardboard, foam-core, plywood etc to channel it to where you need it - effectively you need the soft-box light to "grow" vertically, to cover a figure, but not grow horizontally, so we can put flags the side of the box rather than using a grid, which constrains the light in both dimensions. Think of the flags a bit like massive barn doors :-/
These examples were made in SetALight3D btw - which I highly recommend if you want to try out different lighting and don't have studio, or a subject handy.
Hope this helps - it seems complicated but it becomes second nature with practice.