Open iPhoto by clicking its icon in the dock.
Double click an Event to view the pictures inside it.
Select the picture you would like to resize by clicking it once.
Select Export... from iPhoto's File Menu.
Select JPEG from the Kind dropdown menu.
Select Custom from the Size dropdown menu.
A new Max Dropdown Menu will appear in the window.
From this menu select either Width, Height, or Dimension from the dropdown. In the entryfield below enter the max pixels to resize your image to. So if you select Width from the Max Dropdown Menu and input a value of 640 into the entryfield, your image will be resized to a max width of 640 pixels.
Click the Export button to resize the image.
A popup window will appear asking you to name the exported file and select the location to save it to. Type a name into the entryfield then select a location to save the file. Click the Ok button to complete the file export.