Keep it short and sweet: a guide on the length of documents that you provide to Copilot

When you write Copilot prompts, you can provide Copilot with information or samples  - the source for Copilot to use.  For example, you can ask Copilot in Word: Write a marketing plan outline based on the product features described in this document. But to get the best output from Copilot, it’s important to consider the length of the content you provide to Copilot, as there are limits to the size of documents that Copilot can read at once.

Depending on the type of tasks you give Copilot, there are times that Copilot will focus only on the beginning of the document (up to its limits) and then ignore anything beyond that. For example, when you ask a question about a specific topic in a long document, document length typically will not impact the ability to get a result based on how the Large Language Models (LLMs) reason over the content. But if you ask for a summary of a long document, the task requires the entire document context to inform the output. For the latter case, length limits do come into play and impact output.

For scenarios where the entire document context is needed for Copilot to provide answer, a couple of good rules of thumb on the length of the document you provide to Copilot:

• For summarize around 80,000 words or less is a sweet spot for Copilot to summarize a document effectively.

• Rewrite works best on a document that is less than about 3,000 words.

Keeping files you provide to Copilot within these size limits helps Copilot read and respond to your documents accurately and effectively. If your document is longer than these limits, Copilot might still be able to help, but it'll focus only on the first part of your document. It would be like reading only the first few chapters of a book and trying to guess the rest (which is what Copilot ends up doing in this case).