I do not believe it is a size issue... I played with a couple other SiteMap programs (needing to completely disable the Yoast plugin, (not just it's sitemap generator).
Interesting, the other programs had the same problem... one difference, though, was that I was able to view the source on the other ones... and I noticed the first line was blank. The error says "XML declaration allowed only at the start of the document".
I wonder if WordPress or a stray plugin is putting in the blank line... I have disable many plugins (not all... I just can't)... and have not fund the problem child...
WordPress.org... help! As a multi-user site I have several clients now with Sitemap issues...