Micro.blog Help has instructions for working with Truncated posts on your microblog:

Truncated posts on your microblog
Customizing a theme.
If you write a lot of longer posts, you may want to only include the first part of those posts on your microblog’s home page. By default, Micro.blog will include the full text of each post on your home page. You can use custom themes to customize this.

Read the instructions: Truncated posts on your microblog Customizing a theme. Published April 26, 2019.