there is no hard and fast rule about content length. It varies from 100 to 1000 words depending on the nature of your blog and its content. The important thing is the keywords which you need to focus and try to rank on search engines. The type of blog also defines the word count or length of...
You need to have a good domain knowledge. Try to read other's blogs which can help you know how people capture their audience. The important thing to focus on the keywords and incorporate them in your blog were needed. This would help your blog rank on google.