Reducing the size of the sitemap files was just for myself to make it easier to check the threads included. Don't think this has any effect on indexing.Indeed, and if we can reduce the size of the sitemap files by editing line 193 in Sitemap.php we'd be tempted to try it to see what results we get too!
doesn't look like it would be optimal for my board which would easily go above this limit. latest version is still 2.4.Plans for 2.5: Sitemap index file for websites with more than 50000 pages or XML files larger than 10 MB. Maybe gzip compression for the sitemap file.
You might be interested in this tweet, @Chris D.Google's opinion, too.
Although this doesn't strictly apply to sitemap submission, it does indeed confirm that Google like you to use the rel=next/prev mark up to hint to them related pages on different pages: