I use either Pixelmator Photo on the iPad or Pixelmator Pro on the Mac. They both work well. The only drawback to the iPad version is that it makes its own decision about how much it can upsize the image, so I usually need to crop it and/or downsize it a bit on the Mac before I print. Previously, on the Mac, I used Topaz Gigapixel AI. That was good, though the current Pixelmators are better, but the current Gigapixel is improved -- just haven't upgraded so I can't compare. There are several other desktop apps that are in a sort of upsizing arms race, so there's lots to choose from if you want to get something new. I think Gigapixel and at least some of the others come as both a plug-in and a standalone. (I can hunt up the app names if you're curious.)