In this forum post, https://tranmautritam.ticksy.com/ticket/795975/, you advise to change the functions file to change the staff_tn size. However, this would be overwritten on  theme update. 

Can this function be used in a child theme function file? 

I added: 

function child_theme_setup() {
    add_image_size( 'staff_tn', 300, 350, true );
}

add_action( 'after_setup_theme', 'child_theme_setup', 11 );

To my child theme function file which seems to have worked though I'm not sure it's proper

So the change to staff_tn in the function has affected the thumbnail size used in the recent blog posts widget. It appears these use the same size...can this be edited using a child theme?

http://77.104.156.249/~phaneuf9/

I am using the Recent Blog Posts widget from visual composer and have limited the posts to 4, used blog style 3 and selected "no navigation"

Trying to find what file/code controls this and rewriting it so it doesn't use the staff thumbnail size might be quite a bit beyond me. 

Can you point me in the direction of what file or files I should look at to try to write the new function?

I just added: 

.rp_type03 .post-attachement {max-height: 170px;}

And let the thumbnail overflow be hidden...i couldn't figure out what to edit in the core file, forget about writing a new function in the child theme.