Is it possible to arrive at one big idea for a passage, even if the passage is relatively long? The answer is yes, as long as you are dealing with a unit. Where there is a unit, there is some level of unity. In almost every case (maybe in every case), a book could be treated as a unit of Scripture. Within that greater unit there are sub-units. Perhaps we could call them major sections. Within major sections there are sub-sections. Within sub-sections there are complete thoughts. These terms are deliberately vague to allow for the different genres. But essentially a book is like a Christmas tree. The higher you go, the more is included within your chosen section.
For example, is it possible to determine a big idea for Ephesians 1:13-14? Yes, in the English Bible this is a sentence with a complete thought. (I know that for Paul it was not a complete sentence, but we’ll get to that!) Now if you step up a level, these two verses would be swallowed into 1:3-14. Again, big idea is possible, but would be different than just the last two verses of the section. Then you could step up a level again and preach that section swallowed within a larger unit – perhaps 1:1-2:10? Certainly you could move up to 1:1-3:21. Again, a big idea that would have to encompass more text, but still it could be a preachable big idea. Then you could move up another level and preach the whole book.
The main idea of a shorter passage becomes a point or movement within the main idea of a longer passage.
The important thing to remember in preaching a larger chunk of text is that you still have to deal with a legitimate unit of Scripture. Also, the details in the smaller units cannot be reflected fully in a succinct big idea. Be very wary of over complicating a big idea for a bigger passage. It may feel more responsible with the text, but it will undermine your main goal of communicating the text relevantly to your listeners.