I think I have solved this, for anyone willing to edit the plugin files.
If you go to the plugin in your dashboard and click on “Edit”, a PHP file will open and there are 10 files in a list to the right. Choose the file “baw-manual-related-posts/inc/frontend-noajax.inc.php” to open it up in the edit window.
Copy the entire file to a text editor and save it, just to make sure you can restore it if you stuff something up.
You need to edit two pieces of code, which change the default size of the image and change the width of the “li” which contains each related post.
1. About halfway down there is a filter for “bawmrp_li_style”, which includes “width=120px”. Change this to 150px, or whatever suits (I used 200px).
2. A little further down is another filter for “bawmrp_thumb_size”, which sets the default to “100,100”. Change this to the image size you want – 137,84 in your case (I used 150,75).
Click on the update file button and refresh your page and it should all come out the way you want (though I had to close my site down and open it up again to get the new values to “take”. Then style the elements as required.
It worked for me, hope it works for you.