Changing format of blog posts to 'summary'

Hi, I’m trying to set the formatting for my blog posts so when you go to the page you see just the first paragraph of text and maybe a photo, which you then click on to read the post in full. I don’t want what I currently have: A huge scroll of text.

I went to sSettings > Reading > “For each article in a feed, show…” and clicked on ‘summary’ rather than ‘full page’, which I assumed would do the trick. Wrong. It’s exactly the same both ways.

Can you help?

Thanks