Hello, trotter!
Please explain to me a bit more in detail.
I suppose you mean a short extract from your post on the blog loop page.(Excerpt)
If that is the case, to set posts excerpt length, go to Customizing ▸ Blog ▸ General menu and adjust the Excerpt length using the Excerpt slider.
https://prnt.sc/kye00e
You can even create a custom excerpt following the next steps:
1. Create a new post or edit an existing one.
2. On the post edit screen, click on the screen options tab on the top right and then select excerpt.
https://prnt.sc/kye1wb
3. This will display an excerpt meta box below your post editor. You can now add any text in this box, and it will be used as the excerpt for the post.
https://prnt.sc/kye2bv
Regards