I firmly believe that a blog post should not exceed 250 words. I am voicing this opinion after I read a rather detailed blog post by Viper Chill wherein he stated that the most popular and well read blog posts contained 1000 words.
Yes, I am surprised at that figure however true it may be. 1000 words! My God, that would be an academic essay only students and teachers would be interested in.
If a blog post does not give what the blogger wanted to tell his readers in 250 words or less, then, he is not an efficient blogger. This is my opinion and I am sure there are differences.
Personal blogs can be of any length. Such bloggers write only as a hobby. Pro-bloggers should be able to blog in 250 words to 300 words maximum.