How big can a sitemap be before I need to split it?

The sitemaps.org protocol caps a single sitemap at 50,000 URLs and 50 MB uncompressed. Hit either ceiling and you need to split content across multiple files and reference them from a sitemap-index. Gzipped sitemaps can be any compressed size, but they still have to decompress to ≤50 MB. For most CMS-generated sitemaps, files land at 1,000–5,000 URLs each (well under the cap) split across a sitemap-index — that's the recommended pattern for fast parsing, easier triage in Search Console, and clean coverage reporting per content type. For converters: xmlsitemapmaker.com bounds remote fetches to 25 MB per file but follows sitemap-index files, so a multi-million-URL site is still one conversion.