Heya,
If you want to test it, get a 43~48" and a 60" and see the kind of spread you get at the widest zoom of the speedlite (24mm) and see if it even matters that you got a 60" or larger. It likely won't because the speedlite doesn't have wide enough spread unless you pull it way, way back from the umbrella (and this will take a lot of space, on top of the space of a 60"). This is why a strobe is used commonly with these huge modifiers, because of the spread angle of light coming out of its nose without a reflector, at a position that fills the modifier. I used a 60" the other day with two speedlites ganged up, so that the spread was greater, and the +1 stop of output, and it works (I did it outside even). But I don't recommend it as an every day approach. I did it because I was in a moment where I had to make it work with three adults, full body, outside, and I didn't want to setup several lights, so I went with my bigger modifier and just used more than one light with it (as a reflecting source, black backed). Basically used it as a big 60" bounce surface with two speedlites.
Again, realistically, speedlites don't really fill up big modifiers. I would keep it simple for 1~2 subjects, a 43~48" is going to do that just fine. You'll find bigger is a lot harder to place nicely in a room where you want all the time. It takes up a lot of space to do it unless the room is massive with really tall ceilings.
Groups:
Don't even attempt to do 12 subjects with some umbrellas and two lights. You'll get shadows and uneven exposure every where. Instead, you have to change how you think. This is where you either need a lot of lights, or a huge light source relative to the subjects. In this case, I would just use the speedlites bounced off a ceiling to manage a big lighting session with a large group of people. If this is outdoors, then you don't have that option, and you have to get creative with posing them, rows, etc, so that no face casts a shadow on another face, etc. It can be done with two lights, direct light, from high above casting down onto the group from a tall stand. But it will hard light. So it's better to have it act as fill, not key, if possible or it will look flashed.' This is common though, when doing teams (like sports) outdoor. Again, the key to this is to make sure sun is not in their faces (outdoor) and that your light(s) are not casting shadows onto other people in the group. This is a whole different way to approach lighting, so start small and simple. Don't try to get a big enough light and big enough umbrellas for a group, you approach groups even more simple, and find a way to use the environment and control shadow fill and shadow cast.
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Practice is the way to go. Even just doing self-portraits to play with lighting to see how it all comes together.
You will of course eventually want bigger, stronger lights, and bigger, more controlling modifiers. Been there too. Done it. And what I ended up doing is going backwards, back to using simple speedlites, a single powerful strobe, smaller modifiers, and just managing shadows and doing more fill. I prefer it to using my 60" and trying to muscle through things in small space with huge modifiers. More to setup, more to carry, etc. These days, I keep it simple. I have a fleet of 6 speedlites and a couple of strobes, and a single portable 600Ws strobe for outdoor sun work and I've been using a 10" modifier these days! Everyone has a preference. But you have to do it to form your own opinion of what you like and want. My point is though, don't go buying into huge stuff yet. You may find it simple to just have some fill light on your subjects and won't need crazy stuff. I often use speedlites bare at the subject as fill, and you cannot even tell, it's soft enough and fills enough (key light being very different, harsh, when directly flashed).
Practice practice. Keep it simple. You will do more with a handful of speedlites and no modifiers than a bunch of huge powerful lights and huge modifiers; all of which you have to carry, setup, maintain, replace when they break, and manage wind sails without assistants, etc. You'll see!
Very best,
