BoAF, T:Ind, Size +1 is a single ball, encompassing a volume of approximately 10 cubic paces.
BoAF, T:Group, Size +2 is 1000 balls of fire, each one a single cubic pace in size. (or some division thereof, maybe 10 balls, each 100 cubic paces)
Nothing tricky about it. Your example about targeting grogs is (again) confusing the group of grogs with the Target:Group of your example BoAF. Each one could streak to a different target, but different troupes might require a Finesse roll, others might be ok with them all simply streaking to their own target. That's what group does-- it allows you to break up the effect into multiple Individuals:
BoAF's size IS defined, by the structure. It's:
The Target is individual, so the ball of flame it creates is the size up to an individual for Ignem. That's how it works. The book doesn't define it every time because they defined it at the front of the Spell chapter. When it's different somehow, the text defines it.
And you call it "flavor text." It's not flavor. It's the descriptive mechanics and guidance to how the spell works-- the sample sigils on some spells are the closest to flavor text.
I'm not sure I understand the part about "recipients who do not match the underlying Form." Spells just hit, that's how it works. Finesse rolls are used to target things which do not automatically hit (and which usually are unaffected by the Parma), like the Sling Flambeau spells out of HoH:S. Otherwise, boom, it hits. That's magic. In your IoL T:Group example, your 10 lightning bolts arc out and yeah, if there's more than 10 in the group, it's up to the troupe how that goes. If they're tightly packed, then I'd say they all get nuked. If they're spread out, then just 10 of them light up, or maybe they all get hit, but we reduce the damage a little, so it's +25 damage instead. This is part of that SG/Troupe judgement that goes into adjudicating a spell in play. I once had a very heated discussion about how the effect of an Aurum spell would flow over buildings-- I felt it would pour over them like water, and the SG at the time felt it would flow over intervening alleyways and only spill into larger spaces. The rest of the troupe agreed with him, and play continued.
I would disagree on the point that "there is a huge room for interpretation." It's pretty clear to me based on the citations I've offered. Everything you need is in the first six pages of Chapter 9, and then in the respective Form descriptions afterwards, for the individual definitions.
Ovarwa is suggesting that if you increase the range to sight, then you get these really long, really thin ribbons of burning fire which do the damage all along their length, which would be pretty devastating to people or animals caught in them. I don't know that I, as the SG, would let it start many fires as it got further out, as they're small, hot, momentary threads, but that would be a troupe discussion if the player disagreed. Again, look at page 113:
That's the benefit of Target:Group. You can divide up those 10 individuals of mass a lot of ways.