Great question! Unfortunately, like most great questions, the answer is, "it depends." When evaluating the breadth of a question like your example, I take the following into consideration, more or less in this order of priority:
- How big is the topic?
- Some theological topics have a huge amount of variability within one tradition or group of traditions, while others don't. The doctrine of predestination within Protestantism is a big topic (many different views) but the doctrine of the Trinity within historical Protestantism is relatively small (general agreement on most aspects)
- How different are the groups/denominations you are asking about?
- The greater the historical and theological differences between the various groups/denominations whose views you want, the more likely it is that the question is too broad. Asking for the views of Marcionism and English Puritanism in the same question would generally be broader than asking for the views of Classical and Revised Dispensationalism, because the latter two share much more in theology and history.
- Does the question ask for a comparison or an overview?
- When I see a question that simply asks for the views of multiple distinct Christian groups in a single question, I often ask myself, "Why is this not asked in two or more questions?" Two good reasons for keeping it as one question are:
- Questions specifically asking for a comparison of two or more groups are less broad than if a comparison is not requested, because answers can focus on the distinctions between the groups, and not spell out in detail everything on which they agree.
- Questions specifically asking for an overview of two or more groups are less broad than if an overview is not requested, because it makes it clear that only the most important points need to be addressed. This also emphasizes that partial answers (those that deal only with one of the requested viewpoints, for example) are not acceptable.
Of course, these are general guidelines, and the application of them is going to vary even among close voters who agree with them. Thus it's dangerous to give specific examples, but I'm a glutton for punishment:
- What is an overview of the theories of atonement in Christianity?
- Asking for an overview helps, but the topic is too big and the range of traditions too wide. Close.
- What are the differences in views of Predestination between Catholics, Arminians, and Calvinists?
- Asking for a comparison helps, and so does limiting the scope to a few traditions, but the topic of Predestination is a big one, and there are lots of variations within each of these groups. Close.
- What's an overview of the differences between Luther's and Calvin's views of the atonement?
- Atonement is a large topic, but by narrowing down to specific thinkers this becomes much more manageable. Asking for an overview of the differences helps as well. Leave open.
- Note that if this didn't ask for the differences between these two theologians, I'd still recommend that the question be split – even Luther and Calvin are different enough to warrant separate questions on this topic.
I see three main guidelines for the questions you are talking about: the size of the topic, the variation between and within the groups being asked about, and the use of more "functional" scoping (comparison, overview).
That said, different close voters will have different judgments of the size of the topic, different levels of familiarity with the groups in the question, and different opinions on how much difference the word "overview" makes. So unfortunately it's impossible to say "Questions can be this broad" without actually asking the question and seeing what the community thinks.
And if indeed your question is determined to be "too broad," it's fine to split it up into more than one question, even if the questions only differ in whose view they ask for.
Other related questions: