Hi Adrian,
Unfortunately, there is no setting to only adjust the intro for the main article only (in magazine view).
However you can customize in the file:
.../components/com_easyblog/themes/wireframe/blogs/magazine/layout_3.php
*the layout number could be vary according to your menu magazine layout setting.
and now find the code below that shows in the file.
<p><?php echo $leadingArticle->getIntro(true);?></p>
and change to:
<p><?php echo $leadingArticle->getIntro(true,null ,'intro',30);?></p>
*change the number (30) to your desired length
You're suggested to backup the file that you want to customize beforehand for precaution