feature image size

Hi, are you wanting to show left aligned thumbnails on the main page?

if so, open up index.php and replace

<?php if ( has_post_thumbnail()) : ?>
   <a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>" >
   <?php the_post_thumbnail('category-thumb', array('class' => 'alignnone')); ?>
   </a>
 <?php endif; ?>

with

<?php if ( has_post_thumbnail()) : ?>
   <a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>" >
   <?php the_post_thumbnail('thumbnail', array('class' => 'alignleft')); ?>
   </a>
 <?php endif; ?>

(Also make sure you do this in a child theme to avoid future updates overriding the edits)

Then run this plugin to resize all to 150×150 – https://wordpress.org/plugins/regenerate-thumbnails/

Hope that helps.