One thing you could look at would be posts. You would need a rubric to decide grades. You don’t want a bunch of one line posts. Or simple link posts.
I wouldn’t want word count posts either. (That’d be like those 500 word essays the students always turn in that are total fluff.)
One thing would be to look for a certain number of several types of posts:
1. Link posts, where they discuss a topic they found on another blog.
2. News posts, where they give their opinion or background info, on some news.
3. Personal posts, where they tell something about their lives or their schooling.
4. In-depth posts, where they are expected to have research done on the post. This could be on internet/blog topics or something else that they are interested in.
You would want reasonable grammar and spelling, but not necessarily to focus on that. I try to spell everything I write correctly, but I sometimes misuse grammar on purpose. I wouldn’t be able to do that in a formal essay, but I would consider a blog entry as I envision them, to be a bit more informal.
Another possible idea is for them to find a blog that they find interesting and read through the blog’s archives, writing about stuff they find there. Or maybe commenting on the ideas in them. This would let them see how a particular blog, or blogs, has changed and, hopefully, matured over time- or not.