A client of mine has a VERY, VERY, VERY large WordPress site (over 1 million pages). Is there a sitemap plugin that can handle this (would need to create sitemap indexes)?
Their site have very shallow crawldepth, so they figure a sitemap would help a lot.
Quick search shows that next major update of Google XML Sitemaps plugin will have index-based sitemap.
Overall sitemap is not a guarantee that site will be crawled and I’d also look into other ways to improve (better and more extensive navigation, more internal links, etc).
I wouldn’t have one sitemap with a million pages in it. I would customize the XML Sitemap plugin, and build a sitemap index.
Please note that, by the time you’ve 1M pages on a site, Google should be crawling your site on a very regular basis unless you’re a spammer or delivering content of extremely low quality. (You meaning your customer, obviously.)
This in itself should solve any indexing issue, no matter what your crawl depth might be.
As for sitemaps, check out Arnold’s (the most popular one). It probably allows to split large maps in multiple files.