I don't think that Dreamwidth cuts or hides posts automatically.

When you go to make a post, look on its top menu bar, slightly right of center. The icon has black lines with a wiggly line under it.

You can also just go in the HTML view and type at the beginning:
(cut)

and at the end:
(/cut)

Replace the parentheses with angle brackets to activate the code:
< >

Generally people cut after the first paragraph or few of text, or after the first picture in a set. If it's short, or the pictures are tiny like icons, you don't necessarily need the cut.

Here's where lurking can really help you. Look at a community such as newcomers or use the Latest Things page to see how people are using cuts. You'll get the hang of what is customary here. And if not, well, you won't be the only person not cutting things. Some folks just don't use that feature.