Related Posts Thumbnails custom thumbnail size, quick change

The plugin is using the thumbnail size 150 by 150 from my media settings theme thumbnail size. This works great for the rest of my website, but I want to change the thumbnail size of the related posts to 90 by 90 and keep my themes thumbnail size the same, is their any way I can edit this code or any other code to get my desired results?

The code for the plugin:

/* Calculating sizes */
if ( $thsource == ‘custom-field’ ) {
$debug .= ‘Custom sizes;’;
$width = get_option( ‘relpoststh_customwidth’, $this->custom_width );
$height = get_option( ‘relpoststh_customheight’, $this->custom_height );
}
else { // post-thumbnails source
if ( $poststhname == ‘thumbnail’ || $poststhname == ‘medium’ || $poststhname == ‘large’ ) { // get thumbnail size for basic sizes
$debug .= ‘Basic sizes;’;
$width = get_option( “{$poststhname}_size_w” );
$height = get_option( “{$poststhname}_size_h” );
}
elseif ( current_theme_supports( ‘post-thumbnails’ ) ) { // get sizes for theme supported thumbnails
global $_wp_additional_image_sizes;
if ( isset( $_wp_additional_image_sizes[ $poststhname ] ) ) {
$debug .= ‘Additional sizes;’;
$width = $_wp_additional_image_sizes[ $poststhname ][ ‘width’ ];
$height = $_wp_additional_image_sizes[ $poststhname ][ ‘height’ ];
}
else
$debug .= ‘No additional sizes;’;
}
}
// displaying square if one size is not cropping
if ( $height == 9999 )
$height = $width;
if ( $width == 9999 )
$width = $height;
// theme is not supporting but settings were not changed
if ( empty( $width ) ) {
$debug .= ‘Using default width;’;
$width = get_option( “thumbnail_size_w” );
}
if ( empty( $height ) ) {
$debug .= ‘Using default height;’;
$height = get_option( “thumbnail_size_h” );
}
$debug .= ‘Got sizes ‘.$width.’x’.$height.’;’;
// rendering related posts HTML
if ( $show_top )
$output .= stripslashes( get_option( ‘relpoststh_top_text’, $this->top_text ) );
$relpoststh_output_style = get_option( ‘relpoststh_output_style’, $this->output_style );
$relpoststh_cleanhtml = get_option( ‘relpoststh_cleanhtml’, 0 );
$text_height = get_option( ‘relpoststh_textblockheight’, $this->text_block_height );
if ($relpoststh_output_style == ‘list’) {
$output .= ‘<ul id=”related_posts_thumbnails”‘;
if (!$relpoststh_cleanhtml)
$output .= ‘ style=”list-style-type:none; list-style-position: inside; padding: 0; margin:0″‘;
$output .= ‘>’;
}
else