It isn't immediately obvious how you get photos out of iPhoto for use with other applications, let alone resize them. I didn't think to look in Export for this feature.
Select the album or images within an album you want to export then select File | Export.
You'll be presented with this dialog - just choose "Scale images no larger than" and type in a width or height. It will preserve the best aspect ratio for each photo.
You can also choose the QuickTime tab to export photos to a QuickTime slideshow as well.
iPhoto is not without its trials but this is a good feature. There just needs to be a button to do this so it's more obvious.