I just got my enlargement (30x20) back from Mpix and they look great. For those of you who are experienced with Aperture 3 then this is probably meaningless to you. But for those of us who are new, it might be helpful info. I needed to get a picture with 2000x3000 pixels created in order for the picture not to look blurry. The "JPeg-complete" that is saved routinely in A3 does not meet these dimensions and if enlarged would be blurry Plus the Image Quality is set at 10 and the dpi at 72. since my image was a horizontal picture I right clicked on the picture, clicked on Export>Version and hit the drop down menu and clicked on Edit. I hit the "+" and created a new file. In this case I named it "20x30H" (I created another one 20x30V for vertical). On the "Size To" bar drop down menu I chose "Fit Within (Pixels)" and typed in 3000 for width and 2000 for height (I obviously did the opposite for my vertical file). I did this rather than use inches because when I tried that the image sent was still inadequate for a 20x30 print-it did not have enough pixels. I slid the Image Quality bar all the way to 12 and changed the dpi to 133. Now if I want to have a 20x30 print done I have my export parameters already set and ready to go. The pictures came out fabulous and I am a very happy camper. So if this helps anyone else in a similar situation then I am glad. In this case I was using an on line printing service-in this case Mpix. If you are using any others you may want to check on their particular requirements for the varying size enlargements and create your own special export for that particular service. Cheers.
EDIT: Oh, don't forget to check your picture for any defects, dust spots, etc. Go to full image (type "f"), then Zoom (z), then look over the entire picture. if you see a defect bring up the Retouch tool (x) and make the correction. This is a very important step if you a making a significant enlargement.