I am using Lion and don't know if this works on previous operating systems. This isn't Photoshop but you can get by.

Once you have exported/saved/dragged the picture out of iPhoto. (I just drag it to the desktop).

1. Click open/it will open in preview

2. Go to tools

3. Find adjust size and you are in business

For additional iPhoto details and support: http://www.apple.com/support/iphoto/