1. In iPhoto, go to the image and click on it so it has a yellow border around it
2. Go to the File menu and choose Export
3. On the little window that opens, choose the kind of file (like jpg), the quality and then choose the Size drop down menu and choose Custom.
4. Select the dimension you want (maximum width or height) and type in a number for number of pixels
5. Click Export button and follow the prompts for where you want to save it, like the Pictures file.