A family of Microsoft word processing software products for creating web, email, and print documents.
Refer to https://www.reddit.com/r/Grammarly/comments/1pik81f/word_for_mac_and_grammarly/
We can confirm that Grammarly for Microsoft Word on Mac was recently discontinued. If you’re using macOS 10.15 or newer, the best option now is Grammarly for Mac, as it integrates directly with Microsoft Word and many other desktop apps, and it’s designed to handle longer documents much more reliably.
To clarify what’s happening in your case: The deprecated add-in (Grammarly for Microsoft Word on Mac) no longer works properly (disappearing) and cannot process documents longer than 150,000 characters with spaces. This is why you are seeing the "too long" message. Grammarly for Mac is the supported product moving forward, and it handles large documents much better.
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hth
Marcin