That area already holds a medium screenshot, which is about 400x300 or something.
But it is responsive, meaning it takes up as much space as available until it maximum dimensions. If there isnt enough room, it shrinks.
I suggest you do not remove that functionality.
If you want to make it bigger, the only way is to increase the surrounding col size in table_top_row.html and table_top_row_premium.html
HTML:
<div class="col-md-4 col-lg-2">
<a href="{$list_url}/{$url_helper_a}details{$url_helper_u}{$username}{$url_tail}" class="hidden-xs">{$screenshot}</a>
</div>
changing the col-lg-2 to col-lg-3 makes it about 300px width
changing it to col-lg-4 it will be the full medium size screenshot
if you change those lines you also have to change the col setup following after the screenshot
HTML:
<div class="col-md-8 col-lg-10">
if you make the screenshot col-lg-3, this one should be col-lg-9
if you make it col-lg-4, make it col-lg-8
( col setups inside rows should never be bigger than 12 added together )