Note: This is an Article that was created on the Microsoft Answers site that I saved so it could be edited. Despite appearances, it is not on a Microsoft site, now.
See my response in the Super User Forum to this question about Maximum File Size.
Some tips to maximize the ability of Word to handle a big document:
When you can, work in Draft View rather than Print View. The control to do this is under the View tab. Turn off background repagination. Turning Off Background Repagination (Microsoft Word) by Allen Wyatt
Learn how to use styles for your formatting, and then actually use them. You want virtually no direct formatting.
Importance of Styles in Word
Tips for Understanding Styles in Word by Shauna Kelly
Managing Word Styles by Suzanne Barnhill, MVPUse the built-in heading styles to organize your document. Use the Navigation Pane and / or a Table of Contents to move around.
Why use Microsoft Word’s built-in heading styles?
Moving/Reorganizing Pages in Microsoft WordUse page-break before paragraph formatting in the heading style you want at the top of a new page. Do not use a manual page break or the Enter key
Word Doesn't Know What a Page IsDo not use the Enter key for vertical spacing - avoid empty paragraphs
2.2 Why you should press Enter only once to end a paragraphNo unnecessary Section breaks
The main reason I see for these is changes in header or footers that could have been handled using a StyleRef field.
StyleRef Field
Header/Footer Settings RecapMinimize the use of floating graphics. Put them in-line-with text when you can.
Keep at least two backups, at least one not on your computer's hard drive.
See: Save in two places by Graham Mayor, MVP and Save Numbered Versions by Graham Mayor, MVP
You should find Word more responsive in Draft view.
Lots of RAM on your computer will also help, probably more than it came with. It is usually one of the least-expensive upgrades. I would say that 16Gb is a minimum although Word works with much less. (I use 32 Gb.) Working with both the document and Word on an SSD can really speed things up. Do not work with a document on a flash drive / memory stick.
A hard limit is 32 Mb of text. Word can open a file up to 512 Mb total file size.
You exceed these at your peril. When you exceed the limits, Word will not open the file and it may be unrecoverable.* There is no limit on number of words, per se. Images and document structure can add extensively to the document size without exceeding this limit. Several things like bookmarks and fields can number in the billions in theory. Operating parameners of Microsoft Word How those number could be reached within the 512 Mb file size, though exceeds my imagination.
The biggest component to add file size will be images,
especially when you edit them in Word. Macros/vba can add to file size
but it will not be significant. For instance, here is a comparison of
two versions of the Normal template, both without any text or images. 
.
The larger version contains more than a thousand macros divided into 53 modules. This adds less than a Mb to the file size. I do not know if the macros count toward the 32Mb limit. It also contains a couple of AutoText entries.
That answers your question. Let's talk a bit more about about accomplishing your objective to create a large document in Word, though. As files get larger in Word, they become sluggish and harder to edit. There are things you can do to make large documents more manageable, though.
Like a physical Encyclopedia is split into volumes and Wikipedia is split into web pages, to do a full Encyclopedia in Word, you would want to split it up. Have you ever worked with a single-volume unabridged Oxford dictionary?
DO NOT USE THE MASTER DOCUMENT FEATURE.
Word can handle very large documents in a single file.
If you want to combine your parts into a single file for some reason, convert to pdf and use Adobe Acrobat or another pdf editor to combine the files.
You can make your larger Word document more manageable by:
Styles are arguably the most important feature in Microsoft Word.
Avoiding manual page breaks. Instead, use page-break-before paragraph formatting in a style.
Avoiding unnecessary Section breaks. If you want to have headers and footers that track page content, use StyleRef fields.
Making use of the Built-In Heading Styles to organize your document. (See also Moving/Reorganizing Pages in Word.) This allows you to maximize the utility of the Navigation Pane and Tables of Contents.
Having any automatic numbering of headings, etc. tied to styles following the directions in Shauna Kelly's Page on Outline Numbering. The same applies to multilevel bullets.
Avoiding empty paragraphs. These add complexity and make editing harder. To space paragraphs vertically, use the space-before and space-after paragraph formatting in the styles you are using.
Running on a beefy computer with 16Gb+ of RAM and both your document and Word on an SSD Drive.
Whenever possible, placing images in-line with text.
Any time you are thinking you want to use a Text Box (Insert > Text Box) explore use of a Frame or a Table instead. Word runs more smoothly with these constructs.
Following the above steps will make your document file smaller and easier to edit and use. Using these principles, I've edited documents that exceed 5000 pages of text.
Unless you need new features, using Word 2003 or Word 2010. Both are out of support, though. Every new edition of Word becomes more bloated with features you may not need and which will slow down performance. The .docx file format is preferable to the .doc format. It is smaller and much more recoverable.
Again, work in Draft View with background repagination disabled.
When writing new parts, considering creating the new part as a separate document, using the same template and styles. Then, when almost complete, copy and paste into the main document.
Again, avoiding use of the Master Documents "Feature".
Making regular backups, including to media stored off-site. That way, if you do exceed the 32 Mb limit, you will be able to use your backup rather than possibly losing your work.
Not editing, viewing, printing your document when it is on a USB Flashdrive/memory stick. I would also avoid editing in cloud storage, stick to your hard drive when editing.
All of the above links (except to Shauna Kelly's pages and the operating parameters) are to pages on my website that expand on these ideas.
** I expect that an oversize document in .docx format could be opened in a text editor or and XML editor and pared down. That would be a huge task, in itself, though.
See also:
