in the end result, you will have two pictures, the orignal in iphoto and the one wherever you save the resized image,
go to file->export and when the window pops up, go to kind and change it (default won't let you change the size) and then in size do custom and a dimension box comes up and you can change the dimensions to what you'd like there. after export you can just put it back into iphoto even with the same name, it'll just be a different size. you can always make a new album, orignal photo, and one for resized photo.
Alex