Hello Evan and Katelyn, I’m Igor, the developer, nice to meet you :).
I love the style of your channels and I’m now going to give you not only the solution but also a general guideline useful for further small changes like this.

As you probably already noticed, the template files of the theme are divided in 2 levels:
Tier 1: the main single pages and archive pages, in the root of theme directory
Tier 2: the smaller template parts, loaded by the first ones, which are all in the folder /phpincludes.

You will find the name of the demplates kind of descriptive about their features.

As general idea, you can override main tempalte files or the included template parts by placing a copy of the file in the child theme folder.
If you want to override a file in the /phpincludes folder, you have to create a /popincludes folrer in vlogger-child.

In this way, you can customize your template parts, like I’m going to explain below, and place it in vlogger-child/phpincludes, in this way the custom file will be safe while updating the theme.

THE IMAGE SIZE:
Regarding this, you can customize it by editing each one of the template parts you want to use, by looking for a function called “the_post_thumbnail_url” inside of each template file.

As theme trick, you can know the name of the template file by checking the class of the elements, to make it easier if you want to edit any detail:

https://imgur.com/a/ql67k

In your case, please edit

vlogger/phpincludes/part-archive-item-video.php

and replace this

the_post_thumbnail_url( 'medium' );

with this

the_post_thumbnail_url( 'large' );

Then please remember to save a copy in the child theme /phpincludes subdir.

— a note about the size —

Actually “medium” is the default to save performance, as my collegue explained, in case of large grids of items. Please remember that setting all the templates on large may result in a slower page loading time, so I’d recommend doing it only where it’s needed 😉

Thank very much for your time, I hope that my explanation was clear enough otherwise please let me know.

Looking forward for your feedback 😉
And thanks for the rating too!

Regards
Igor

PS. Sorry for the late reply, we just had a baby 3 days ago, so these days I don’t have so much time to spend at the computer. Sorry for this, usually we provide a much faster support 😀