I got the same problem as Mr Eddy in this thread describes:
I want YARPP to use the already existing 150x150 thumbnails and resize them to 120x120px instead of using the original size and resize them.
I did exactly what was descibed in the post above. At least I think I did. ^^
I put this code into the function.php right before "?>":
add_image_size( 'yarpp-thumbnail', $width = 150, $height = 150, true );
I also used the recommended plugin to regenerate the thumbnails, but nothing has changed.
The result can be seen here (I didn't resize them yet, because I wanted do make sure YARPP uses the right picture)
I hope I could point my problem out well enough. I am not a nativ speaker.