[support] Thumbnail resolution for CCK field

I would like to know where and how to set the resolution of the thumbnail that is generated at the time of uploading an image. The image populates a CCK file field using the image widget. I would like to be able to set different resolutions for different fields within the content type. The particular case I'm working on is a letterhead field that contains an image. I have a separate theme for presentation on handhelds and mobile phones. I would like to use the thumbnail on those pages but the default resolution is too small. I'd rather not send the full image file to a phone nor calculate resizing values for speed reasons.

Thanks, Scott