Hi. Confluence accepts about 5MB of content on one single page (this excludes any attachments). That are roughly 800.000 words.
If you run into problems when saving very large pages, you should save the content in several smaller pages.
Unless it is a strictly internal page, it’s always recommended to keep the pages smaller and with that more user friendly.
Here you find more about pages: https://confluence.atlassian.com/doc/create-and-edit-pages-139476.html