Longer copy is acceptable. Including the full blog post text means a longer email, but there is a benefit to including the full blog text for your reader. You are offering them convenience by not making them click through to your site to read everything. You hold nothing back and give them everything for free, not even requiring a visit to your site. That can be seen as a form of goodwill, and readers often prefer the convenience of being able to read everything in their email client on their mobile device without having to open up a browser.
Topical trends: See if there is a trend across these posts that make for a singular topic. Unrelated blog posts might still have a connection. For example, on my personal blog, two high-traffic posts have to deal with customer service issues with Dell and Verizon. They are unrelated except that they indicate people are looking for help in dealing with large corporations when they are frustrated. That trend could be a topic. What topics are people reading that are related on your blog?