Changing the post excerpt length (teaser text) of your theme is relatively simple. You will need to edit the /content/themes/mytheme/partials/loop.hbs
When opening your loop.hbs
file you will see code like the this:
{{! This is the post loop - each post will be output using this markup }}
{{#foreach posts}}
<article class="{{post_class}}">
<header class="post-header">
<h2 class="post-title"><a href="{{url}}">{{{title}}}</a></h2>
</header>
<section class="post-excerpt">
<p>{{excerpt words="40"}} <a class="read-more" href="{{url}}">»</a></p>
</section>
<footer class="post-meta">
{{#if author.image}}<img class="author-thumb" src="{{author.image}}" alt="{{author.name}}" nopin="nopin" />{{/if}}
{{author}}
{{tags prefix=" on "}}
<time class="post-date" datetime="{{date format='YYYY-MM-DD'}}">{{date format="DD MMMM YYYY"}}</time>
</footer>
</article>
{{/foreach}}
You can change the default Casper value of 26 to any value you want:
{{excerpt words="26"}}
You can play around and change this value and see what length best suits your writing style and theme.