Not sure if the iPhoto app does this but when you email a photo from the Photos app you are presented with several size choices. If iPhoto doesn't do this, use iPhoto for iOS to share your photo to the Camera Roll and email it using the Photos app.
You do not see the option to size the photo until you select photos to share, then select email. The option will appear then. There are usually several sizes offered. Sometimes you have to scroll down to find Actual Size. Each option will tell you the file size of the photo.