Zenfolio is partnered with several print shops. Which one(s) did you decide to use for your fulfillment? If you chose MPIX I'd go to the MPIX site and see what their recommended settings are. Again, like John said, many printers ignore the DPI in the file and set it according to their standards. 300 DPI is somewhat of a standard so I'd feel pretty good about choosing that. I would simply upload the full resolution files so that they will achieve the highest quality once they have been up-rezed for poster size prints. I do however think it's a good idea to offer a low-rez file for smart phone, email, social networking, etc. I think Zenfolio will do the heavy lifting for you though. I don't think it's necessary to upload the same file twice in different resolutions. I "believe" that Zenfolio will down-rez your file for digital downloads.