Hey you have to edit single-post.html file. And find the following code.
<div class="author-box">
<div class="medium-2 small-2 columns">
<a href="#">
<img alt="" src="images/help/author.png">
</a>
</div>
<div class="medium-10 small-10 columns">
<p><a href="#"><strong>About John Doe</strong></a><br> Lorem ipsum dolor sit amet, consectetur adipiscing elit. Morbi at velit in neque efficitur vehicula. Pellentesque id mauris vestibulum, ultrices eros ac, hendrerit ligula. Ut magna felis, tincidunt eget finibus a, finibus at purus. Nulla viverra interdum est, quis interdum nisi ornare eu Cras fringilla.</p>
</div>
<div class="clearfix"></div>
</div>
Now in this code change medium-2 to medium-3 and medium-10 to medium-9
This will increase the width of both columns. Once done then you can add this CSS into style.css
.author-box .medium-3 img {
width:200px;
height:auto;
}
And your author box , image of author width would be 200px. Let me know any confusion thanks.