Krunch, as far as I can see you've already found the solution. We would really appreciate if you share it with us.

P.S. I suggest that you try using NetBeans IDE: https://netbeans.org/
This development environment will help you to find the functions and methods you need easily and fast.

Not 100%, there is no Div ID so width and height can't be adjusted to needs.

Thanks will download and study this Netbeans.

Nothing yet on locating where <div style="width: 75px; height: 54px; background: url('.$thumb.') no-repeat center center;"></div></a>';  may be found?

Found those codes at:
/public_html/ow_plugins/video/init.php
/public_html/ow_plugins/ivideo/init.php

Made needed changes, set debug, cleared cache, refreshed a few times, No-Go still..

For now I was able to accommodate the desired video thumbnail-size by applying codes at admin Css as follows:

.ow_newsfeed_item_picture div {
background-repeat: no-repeat !important;
min-height: 100px;
min-width: 140px}

Then uploading my custom image to theme image folder, hoping for proper fix soon..

 

Krunch, you are right, the files which are responsible for the width and height are:
/public_html/ow_plugins/video/init.php
/public_html/ow_plugins/ivideo/init.php

But, the changes you've made will be applied to the new items only. Since the old item has already been submitted to the database, you should make the changes via phpMyAdmin >> ow_newsfeed_action table.

Note: This solution will work for the 1.5.3. version.

Yes, good reminder, thanks Daisy.

Also if you have used any other theme in the past to make changes to image format at that theme's Css file accordingly from .jpg to .png,,if in fact your custom video-no-video thumbnail happens to be as .png,, again thanks Daisy..