Reply To: two indd docs, same text, different type size for each doc, shared editing

Easiest way to do it would be to use a Word doc and set up Preferences to link to it. (See “Create Links when Placing Text” checkbox in File Handling panel of Preferences dialog box) This will only work if you only make edits in Word, set up your para styles very carefully, and not edit or format in InDesign after placing.

WordsFlow is an even better (commercial) solution with much for flexibility:

Import Word Documents, Edit in Word and InDesign, and Keep Linked for Updating

Another option is to use the Edit > Place and Link feature. This isn’t a well-understood feature, but it can be quite powerful, and even works across two documents, like what you’re describing. Alan G wrote about it here:

Linking & Syncing Content in InDesign