How large? 32 Mb, roughly. I have documents that exceed a thousand pages.

If you are working with a large document, minimize direct formatting. Apply
formatting through styles. If you are inserting graphics, do so as links
rather than by embedding the documents. Make sure Fast Saves, Versions, and
track changes are turned off. Save regularly and keep backups. Never, ever,
ever, not even once, save to a floppy.

As Charles says, the stated file size limitation for a Word document is 32
MB, but that's for the text alone. If you have embedded graphics, it can be
much bigger. Pagewise, some users claim to have created Word documents
(presumably text only) upwards of 10,000 pages. It helps to have a lot of
RAM (that's more important than a fast processor), and, as Charles says,
consistent use of styles greatly reduces document complexity and size.

If you're trying to keep file size down, note these causes of file bloat:

1. Fast Saves: Disable this at on the Save tab of Tools | Options.

2. Preview Picture: Clear the check box on the Summary tab of File |
Properties.

3. Versions (File | Versions): Make sure "Automatically save version on
close" is not turned on.

4. Revisions (Tools | Track Changes):
   Highlight Changes: Make sure "Highlight changes on screen" is turned on.
   Accept/Reject Changes: If "Accept All" or "Reject All" is available then
revisions are present; accept or reject all changes, then turn Track Changes
off.

5. Embedded True Type fonts (Tools | Options | Save); embedding fonts should
be avoided wherever possible.

6. Embedded graphics: When feasible, it is preferable to link the graphics.

7. Document corruption: See
http://www.mvps.org/word/FAQs/AppErrors/CorruptDoc.htm.

--
Suzanne S. Barnhill
Microsoft MVP (Word)