For Website/Blog Description: You should be indentifying your blog or website description. When you purchase/buy your site or domain you think site title and purpose then write your description about your site. You must take your site description related to your site name or title. Don’t forget to put your site title at site description.
I hope it is best idea to get site rank and top position at search engine. Site description need to give 150 to 150 words without space. Don’t give more than 155 words. I think its better 152 words to give at site description to reached top position.
For Post Description: Post Description is more important for get top position at search engine. If you are aren’t follow or maintain SEO for Post Title and Post Description. You don’t get much more visitor. If you want to get more visitors, must be thinking your post title and description, according to the focus keywords. Give some more valuable keyword at post description which is having at your post title and Tags.
You write your post description 200 to more words. At list write 250 words with space because 250 words is the best, you can give more words that also better.