resize photos in iphoto

Most of my photos off my camera are 1944x2592 1.5/1.7 MB.I want to burn them to a disc .I usually have them printed off at 4''x 6''(local self serve)the printing machine tells me the resolution will not be right for that size.Itried to resize in iphoto File,Export.format Jpeg,scale to specific size width/hight ,when i try to change size to 1024x768 it won,t .
I wanted to use this disc as a back up disc to save my photos .

IPhoto doesn't actually resize photos (except by cropping). Unless you're only taking photos to be posted on websites, you would NEVER want to resize your originals down to 1024x768@72dpi. That's barely acceptable resolution for an Ebay photo.