Image Size

In order to change add_image_size() of theme defaults (popcorn-child/functions.php):

<?php

//Change default video thumbnail size in core similar to hentaifox.com with css aspect-ratio: 2 / 3
add_action( 'after_setup_theme', 'popcorn_child_theme_image_size', 20 );
function popcorn_child_theme_image_size() {
remove_image_size( 'thumb-video' );
  add_image_size('thumb-video', 240, 340, true);
  //you can change add such as below theme defaults:
  //add_image_size('forum-icon', 64, 64, true);
  //add_image_size('model_profile', 120, 170, array( 'center', 'top' ) );
  //add_image_size('model_profile_large', 160, 260, array( 'center', 'top' ) );
  //add_image_size('category_image', 250, 140, array( 'center' ) );
}

Above function will crop images vertically so you will need also a css code to change thumbnail aspect-ratio: read more about aspect-ratio property

.thumb-view .thcovering-video img, #rotator img {
    aspect-ratio: 2 / 3;
}

Changing number of columns of video previews in popcorn is too easy:

.videos {
    grid-template-columns: repeat(5, minmax(0, 1fr)) !important;
}

Simply change the number 5 which is default to x number you wish and you will get x columns on the frontend.

Change margin between previews:

Space between preview images by default is 1px you can give to .videos class a grid-gap command and go.

.videos {
    grid-gap: 10px !important;
}

INFO: CSS code goes to theme settings panel Additional Css section that saves in database or directly in popcorn-child/style.css file, you have to clear all existing caches like cloud flare, plugin or browser history.