There is a limit to how large a site can be and how many pages there can be within a site.
Google Sites (new): Up to 200,000 characters per page, and up to 1000 pages (max 10 million characters). See https://support.google.com/drive/answer/37603
Expect to have problems if you have too much content or too many pages in a site.
For users looking for a much larger structure, it is best to create multiple sites, then use custom URL links in the navigation to link to the distinct logical sites.