change filesize of iPhoto image before adding it to a mail
If you click the email button in either Aperture or iPhoto, or for any photo you drag into Mail, you'll find a drop-down at the bottom of the window once you select an image that allows you to change the size of the image, and it tells you the size of the resulting file.

Small, Medium, Large, and Original are the options.

As I said, this works for any image in Mail. It won't affect the originals in any way. It's the easiest way I've found to change the images, and it tells you the results on the fly, so you can adjust each image as required.