Changing thumbnail size – Please give example

According to the FAQs, to change the thumbnail size on my blog I’m supposed to add the following line to my functions.php file:

add_image_size( ‘yarpp-thumbnail’, $width, $height, true );

And I’m supposed to give the width and height. Unfortunately, without an example I don’t know how to do this. I’ve already broken my functions.php file once trying the following:

add_image_size( ‘yarpp-thumbnail’, 120, 145, true );

Can anyone give me an example of how to get thumbnails with a 120px width and 145px height?

http://ww.wp.xz.cn/extend/plugins/yet-another-related-posts-plugin/