Some marketing gurus say "the shorter the better" (people don't want to read long posts -- show them some sizzle and then move them along to whatever you're selling).
SEO gurus tend to say "the longer the better" because that way a post will vomit up more terms for the search engines to lock onto and rank the page for.
Blogger opinionators of all kinds tend to say "it depends." I concur.
"Blog" is nothing but a publishing format/content management convention -- reverse-chronological posting of content. Its early usage was for personal journals, but these days the format/convention is everywhere, in use by publications of every type imaginable. Wordpress and other blogging applications have become full-featured content management systems.
So, how long your blog post should be is a function of what you're trying to accomplish.
I've published (as mentioned above) more than 100,000 blog posts at Rational Review News Digest (more than 11,000 in its latest Wordpress database, which will turn one year old on Friday, the day before the publication itself turns 15 years old in its current format). Those posts have a preferential length of 150 words or less and a semi-hard limit of 200 words. Why? Because they are excerpts from and links to Other People's Content and we want to stay in "fair use" territory.
At The Garrison Center, my posts invariably run between 400 and 500 words in length because I'm actually writing for the newspaper op-ed market and that length range has, in my experience, proven itself the "sweet spot" for working that market.
Here at KN@PPSTER, anything goes as the spirit moves. Sometimes I link to something I like with a short comment and the post comes to fewer than 100 words. Other times I reprint an old article or essay of mine that was originally published elsewhere and the post comes in at 3000+ words.
So, form should follow function. If you have a good reason to pay attention to post length, by all means do so. If not, don't sweat it.