When considering scope, you can always start with two basic questions:
- how many people are involved?
- how many people are affected?
If the scope of the conflict is too small - only one person is involved, and nobody is affected - you may find readers going "so what?" You can make a story work with a conflict that involves and affects only one person, but in that case it will be very important to answer the question of why it matters that this person get through the conflict. Maybe in the case of personal moral dilemmas the significance comes from questions about the nature of human morality - the individual symbolizing us all, and thereby giving the small scope a larger meaning.
On the other side of things, you have the save-the-world/universe conflict, in which you have a large number of people involved, and absolutely everybody is affected. The number of people involved will be somewhat limited by what the narrative can bear without confusing readers. In The Lord of the Rings, we're dealing with all of Middle Earth falling into darkness, and so many people are involved that the narrative splits in order to deal with them all. Furthermore, almost anyone can be enlisted from the population of passersby to act on one side or another of the conflict, because it affects all of them.
In the case of The Lord of the Rings, everybody knows about the conflict and it's easy to get everyone involved. In the Harry Potter books, I found it interesting that the scope of the conflict kept increasing. At first it seemed like just Harry was involved, and maybe a few more people. Then the further we went the more it became clear that all of the wizarding world was involved, and by the end we were starting to see that even the Muggle world was involved. Good stuff.
But if we're to talk about potential problems with the scope of story conflict, I have to mention the Harry Potter books here too, because I had a quibble with Harry Potter and the Order of the Phoenix on the level of scope. I entered that book with the impression that the conflict would center on Harry and his friends as both the primary people involved, and those affected - but it turned out that there were many more people involved, and that Harry and his friends were occupying a small corner of the affected area, in such a way that I wasn't convinced there was any way they could actually have resolved the conflict on their own. This isn't something that will bother everyone, but it bothered me at the time.
If the scope of your conflict is too large, readers may be confused. Make sure that you're showing all the people involved, and taking the necessary steps to imply the number of people affected. If the scope is too small, relevant only to the main characters and not to anyone else, make sure that you're showing readers why such personal stakes matter.
The reader's understanding of the full scope of your conflict isn't something that should necessarily remain the same, either. Expanding the scope of the conflict - at least in terms of the number of people affected - is part of what raises the stakes as the story progresses. You can plan ahead for moments that reveal expansions in scope: that moment when suddenly you realize that more people were involved than you ever suspected. Or the moment when you realize that a single decision that rests on one character's conscience will affect everyone you have read about so far.
Of course, the issue is more complex than I can really explain through generalities, so here are a few of my thoughts about scope from my writing of my new novel, For Love, For Power.
I'm finding that I have to keep reminding myself about issues of scope. The main characters are all members of a single nuclear family which has lots of internal struggles, and at the same time, they are involved in a larger-scale conflict over the leadership of the nation of Varin that will affect everyone. Where the question of scope becomes more complex is in the fact that their nuclear family is part of the larger group called the First Family, and that there are twelve Great Families in the nobility, all of whom have a stake in the leadership struggle. Because of this, I have to decide how many of them are involved in events (say, attacks and attempted assassinations, meetings and negotiations) that directly affect the First Family and my characters. Logically, since everyone is involved in the process, the events that affect my characters are not the only ones that are going on at any given time. I'm finding that I have to build in ways for my characters to get information about aspects of the ongoing conflict that don't directly affect them. Otherwise it would appear that the First Family is the only important group here, and then why would the struggle for leadership have any meaning? Thus, if the First Family is attacked, then very likely several other families will suffer attacks on the same day (and some may be initiated by the First Family!). If people who attended a particular event are getting sick, then maybe my small group of First Family members should get a message letting them know how many people are ill and how far it affects the nobility as a whole. At the same time, I also have to realize, and try to hint to readers, that the First Family is not the only group experiencing internal struggles. Otherwise their efforts to affect the First Family would be too effective. In fact, I am planning deliberately to have some of the other Families' efforts to affect the power struggle fail because of internal problems in their group. I guess I would call it a question of making sure that I imply the scope of the type of conflicts and setbacks that the First Family suffers, and not just the scope of the overall power struggle.
It's something to think about.