WP 2.1 how do you chage the default thumbnail size?

I have upgraded from 2.07 to 2.1 but now need to redo my thumbnail default size. In 2.07 I changed it in the inline-upload.php (or something like that). But it seems 2.1 has a few different files and names. Which one do I need to change, oh and I am not a PHP god as most of you are… Thanks!