Hello Mark,
No problem! So, MailChimp sent me this in an email:
"As for limiting the content, that is also dependant on how the feed is coded. As per industry standards, the <description> tag is used to include a short introduction to the blog and <item> tag includes the entire post. However, I have seen it done in a way where the <description> tag hosts the entire blog post.
Taking a look the RSS-Driven campaign in your account I can see that it is setup to pull in the <description> only, however, your blogfeed - ......... - hosts the entire blog in the <description> tag. The best way to move forward would be to reach out to your blog feed provider and ask them to code the feed as per industry standards."