I don't use iphoto yet, so not much help but I did find this:
Click on the picture you wish to resize. Click on "Share" then "Export" in the menu above. In the resulting dialog box that shows up click on "Web" and on the bottom right hand side insert the size in pixels that you want. I would make a copy first before performing this operation.
Resizing My picture [Archive] - Mac Forums
And here:
How to resize an image in iPhoto — Lilahpops :: Plunging into Photography