Please check the plugin’s FAQs.
To change “yarpp-thumbnail” size, add the following to your theme’s functions.php file with appropriate width and height variables:
add_image_size( 'yarpp-thumbnail', $width, $height, true );
When you do this, make sure you also set the YARPP setting “Thumbnail Size” to “yarpp-thumbnail”.

Note: Each time you change YARPP’s thumbnail dimensions like this, you will probably want to have WordPress regenerate appropriate-sized thumbnails for all of your images. We highly recommend the Regenerate Thumbnails plugin for this purpose.
You may also select the “Generate missing thumbnail sizes” option to generate any missing images on the fly. Doing this type of processing on the fly may not scale well for larger sites.
For larger sites, we recommend the Regenerate Thumbnails plugin or WP-CLI to generate missing thumbnail sizes in a batch process.
New images should continue to automatically get all active thumbnail sizes generated when they are uploaded.