Approximately 123 MB to 134 MB, depending on the variant.
Google does not host old versions of YouTube on the Play Store. You can only download APK (Android Package Kit) files from third-party archives. This is where risk enters. download youtube 17.34.35
Since this is an older version, you cannot simply install it over the current YouTube app. Android’s security model prevents downgrading unless you uninstall the update. Approximately 123 MB to 134 MB, depending on the variant