Hi nwalch, thanks for posting.
What iPhone do you use? An iPhone 4 defaults to 1936x2592 pixels, which, at 300ppi, makes an excellent 6x8 inch print. (Have a read of my thread "Image Resolutions and Resizing" for a better idea about this). An iPhone 3 or 3GS will probably give you good 3x5 inch prints, but may start to pixellate at 6x8 inches and upwards.
I've been researching different printing companies and online services. There's quite a disparate range of opinions amongst the ones I've looked at. Some maintain that the photographer/artist should send in their images at the size they were taken...however, I think their comment is aimed at the standard DSLR photographer, who probably uses a 12 Megapixel camera, or higher, and not your usual iPhoneographer with his/her 3-5 Megapixel camera. Other companies say they will resize the print for customers, but don't give any details what techniques they use.
300ppi (now I know there IS a difference between ppi and dpi, but I'm not an expert and you don't really need to go into the specifics of this when you're just wanting to print your photos
...ppi means "pixels per inch", dpi means "dots per inch")...anyway 300ppi (or dpi) is the industry standard for good prints without pixellation. Some companies insist that you send them images that will fit the size you want printed, at 300ppi. So, for example, if you want a 5x7 inch print, then multiply that by 300 = 1500x2100 pixels, to give you the minimum size resolution that company wants. Other companies claim to be able to print without pixelllation down to 100ppi...which, if you take their word, means you can have your print 3 times bigger, i.e your 5x7 inch print can become a 15x21 inch print.
I've sent resized images to a company for printing, however they came back and asked me to send it to them in the original resolution...I think this is because they ran my resized image through their in-house resizing software, which would effectively have tried to enlarge an already large image. And yet another company that I tried e-mailed me back to say, yup the image was lovely and big and they would have no problem printing it at 12x16 inches.
What Apps do you use? If you use a photo editor such as Iris Photo Suite, or Filterstorm, you can resize your images in the app itself, before saving to your PC. So, when you're uploading your image to the online printing service, the image is already resized.
Hope this helps!
Aly