If your post has a lot of content, both words and images, it will run down the page a ways. On the post page, where the post resides by itself, it really doesn’t make much different how long the post is unless your readers have a short attention span.
However, on your WordPress front page you are better off truncating the post with a Read More > link so that the reader can skim the content of the post without losing site of the previous post(s) just below it. Having several posts visible on your front page makes it less overwhelming to your visitor and shows a sense of how much content you have on your site.
Somewhere around 150 to 200 words into your post, click on the “Read More” tool in the top edit menu.
This will put a line across the post indicating where the Read More > link will appear. You can adjust the location of the link so the jump to the post page makes sense to the reader.
Experiment a bit with how this works and then use it with a long post to tighten up the content on your front page.