It might seem like there is a pretty straightforward correlation between sensor size, resolution, and maximum reasonable print size, but the relationship is a bit more complicated than just those specific variables. This great video examines the sort of print sizes you can reasonably create using photos from an iPhone, an APS-C camera, and a medium format body.
Coming to you from First Man Photography, this interesting video discusses the maximum size prints you can create from different sources ranging from an iPhone to a medium format cameras, along with an APS-C sensor. While the source of the images certainly has an impact, it is more than just this. Our own Lee Morris investigated this a few years ago when he shot an image for a billboard and he only needed two megapixels to do so. The issue mostly boils down to viewing distance. If you are viewing a photo up close, you will need fairly high resolution and detail, as your eye will be able to discern a lot more detail. On the other hand, for something seen from much farther away (such as a billboard), less detail is needed. Check out the video above for the full rundown.