What's a good print size for an iPhone 6 panoramic photo?

S a good print size for an iPhone 6 panoramic photo?

That really depends upon the image aspect ratio of your final pano.

If I use two overlapping shots, then the final image aspect ratio can be 16:9, the same as the image aspect ratio of a Full HD video frame (but of course with much more resolution and detail)

I have a shot that I printed 13 x 53 inches using my Epson R1800, about a 4:1 image aspect ratio.

While the iPhone 6 has an 8 mp sensor, it is a tiny sensor, so the colour depth, dynamic range are going to be problematic if you intend to make prints larger than 6 inches on the shortest side.

As you can see in the link below, the iPhone 6 sensor is just tiny when compared to a full frame digital SLR