While reading, I'm finding difficult to understand how to read an initial 大, since it can be read だい (大好物), たい (大使館), おお (大蔵省).
When I can find the word on a dictionary it's not a big problem, I can just check, but when dictionaries don't list it I have no idea how I should read it; for example, 大集団 should be だいしゅうだん? おおしゅうだん? It's the same? I tried Jisho and Weblio, and I wasn't able to find it.
IME gives 大集団 for both pronunciations, but I know it also makes up for errors to some extent, so I can't be sure if both readings are right, or IME is just making up for a mistake in typing.
As far as I found, there are no hard rules in this regard, but since in that answer the answerer speaks about when the word was borrowed from Chinese, and since Japanese speakers have to read it somehow when speaking the word I was wondering if there is some rule about how to read it when used as a prefix of other words, rather than using a word that already contains it.
I know the opening examples aren't examples of prefix だい, I just bring them to exemplify the different readings; my doubt is that, given that as far as I know a general (i.e., not just prefix) initial 大 doesn't follow any hard pronunciation rule, is there some way to know how it should be read when used as prefix, like in 大集団?