My preferred printing operation is Costco. Convenient and very reasonable in cost. Phillippe Callioux, a member of the Palo Alto Camera Club, wrote a blog post in 2009: Getting Good Prints at Costco that is a good link to read now and re-read often. Even if you are printing at home or through another service bureau.
In reading the comments to this post this gem screams out to me:
BrandonBarrett says:So much to learn ...
September 28, 2015 at 12:03 am
"I would highly recommend saving any photo you want to print out as a PNG file, preferably from a RAW file that was edited in photoshop. When you save to JPEG, you lose a lot of detail. If you save as a PNG you keep the full resolution. I was able to upload my PNG image at Costco and have a 20×30 printed out. The file size was 51Mb. I gotta say, the print looks amazing! Very detailed with no noise."
Updated added 8/31/2018
Printing chart for 8 MP and 12 MP iPhone Cameras (MP = megapixel). This chart was posted in our Shared Album for Spring 2018, but that reference will soon be closed. Jill Emmer article https://iphonephotographyschool.com/print-iphone-photos/ > How big can you make a print.