Related Posts Thumbnails

Hi all, I´m using the related post thumbnails to show images in the posts, but when I need to set the option “Custom field name:” to use a custom imagen size of 100×100 the post don´t show the thumbnails.. I don´t see the images.. I´m not sure about what is the problem.. but maybe is the custom name that I need to user in the field of “Custom field name:”.. anybody can helpme? thanks a lot!