Indesign - Document Resize

Not sure if I understand your goal correctly...

One of the methods could be:

Create new document of desired page size. Place your original indd file here, page by page. Resize them to fit (this could be automated in some manner).

This new document is good for exporting pdf's or such. All content edits, if needed, should be done in original file. Save file, update links... Exactly the same procedure as with any other linked objects.

Ah, I see...

Well, your case fits exactly. If I were you, I would go that way I described above. It is fast, reliable, and it works. Just try...

Since your original file has only one page (right?) 'page by page' is irrelevant.

Place exactly as you do with your pictures: File>Place - select your "too big" indd file... Further actions are pretty intuitive, I believe

You're doing all right. As I said, all edits should be done in original file. Your new file is dedicated just for RESIZING - easy and reliable.

It's like an image now... When you place psd file in InDesign, you don't expect have full control on its content here, right? You do edits in Photoshop and update link. Here's the same workflow - do edits in original file (your 2,5x5m indd file) and update link...