We are actually in a chicken and egg situation right now
Some customers are complaining that there are just too many settings while some others like yourself wants more settings. I don't want to add more settings into the component, perhaps we should enforce it to use a large version instead
Temporarily, what you can do is to edit the file /components/com_easyblog/themes/wireframe/blogs/entry/default.php and at line 216 locate the codes below:
<a href="<?php echo $related->getPermalink();?>" class="eb-related-thumb" style="background-image: url('<?php echo $related->getImage('thumbnail');?>');"></a>
Replace it with,
<a href="<?php echo $related->getPermalink();?>" class="eb-related-thumb" style="background-image: url('<?php echo $related->getImage('large');?>');"></a>
If "large" is not good enough for your wide layout, you could use "original" but be aware that this also means that your page loading time will take a longer time to load since the image could be huge?