You can set any dimension you like – the theme generates the required thumbnails if WP’s thumbnails haven’t the right dimensions. Some plugins use the standard sized thumbnails – so I wouldn’t change the default thumbnail size.
Thanks Dude, I have been using a theme with unordinarily wide thumbnails. I just found the thumbnail dimensions in the function.php files and will match the base thumbnail in the wordpress settings. I can’t remember what the default wordpress settings were.