When you are writing, you have to read over what you just wrote - over and over and over and over again. You don't have to read the words aloud (although that is entirely permissible) but you do have to listen to the way the words flow. Do they sound good? Do they make sense? Is there a more descriptive way of saying it? Is the thought simply expressed? Are the sentences short? Are the phrases original or did you go for the hackneyed? Change things and then change them again and then, yes, change them yet again. Be open to dropping the word or phrase that you just fell in love with ten minutes ago because now you have discovered that the word or phrase doesn't work. Set it aside. You will use it some other time. Sometimes you will realize that you started in the middle and you will need to go back and add on the beginning. No worries. It happens. The beauty of word processing - so easy to write the middle before the beginning. How interesting!
Don't be too worried about length. Say what you have to say and then wrap it up. I usually think I am done and then publish the piece but come back to look at it in a day or two. Of course, as you might guess, I want to clean it up again -- and that is how you finally finish the blog post - clean it up and move on.