Gimp question

Dear All, I am using the gimp that came with my Suse 8.2. I would like to save some photos at 300% magnification but I cannot seem to figure out how to do that. Does anyone know how to do that? I know how to magnify but I cannot get it saved at any magnification higher thatn 100%. I probably will want to save them as jpeg at 300% although the extension does not matter that much to me. Any help will be greatly appreciated. Thank you, Marcia