I mean obviously, the smaller the print size the higher the resolution would be. This really is a question of how close up or far away you want the print to be.

If you're doing a metal print I assume this is going to be like wall art; appreciated from a distance but not having your nose up against it?

Googling around, looks like the iPhone SE shoots ~4000x3000 with the back camera. Assuming the photo itself is good, for something like a wall poster and viewing at like 6 feet or more away, you might get away with any of those print sizes.

Something to bear in mind though is aspect ratio. Your camera shoots 1.333 and all of those options are 1.00 or 1.25 or 1.50. So one way or another part of your photo is going to be chopped off. That may play into what size you select.