Well you're going to have to crop that image as 1250 x 1000 is not the same aspect ratio as 800 x 600 - 6 -ish:5 not the same shape as 4:3.
Anyway: to scale an image
Select the image in the iPhoto Window an go File -> Export. In the export dialogue check under the Size option. Under 'Custom' you can set the maximum dimension for the length or breadth.