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>
    <section class="post-excerpt">
        <p>{{excerpt words="40"}} <a class="read-more" href="{{url}}">&raquo;</a></p>
    <footer class="post-meta">
        {{#if author.image}}<img class="author-thumb" src="{{author.image}}" alt="{{}}" nopin="nopin" />{{/if}}
        {{tags prefix=" on "}}
        <time class="post-date" datetime="{{date format='YYYY-MM-DD'}}">{{date format="DD MMMM YYYY"}}</time>

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.