Font size will be different across different devices, due to the differing pixel densities of the screens, but it can be surprising how well the same or similar sizes work across them all. This is largely due to viewing distances. Here’s the pixel densities for the devices:
- iPhones (all except Plus models): 163 points per inch.
- iPhone Plus models: 153 points per inch.
- iPad Mini: 163 points per inch.
- iPad (non-mini models): 132 points per inch.
- Macs: About 110 to 140 points per inch.
- Apple’s external displays: 110 points per inch.
As you can see, the densities typically get a little lower as the devices get bigger. This means the smallest comfortable text size (for me, anyway) is around 10pt on all devices. Around 14pt is pretty good for body text. Bigger sizes are less of a concern for legibility, provided there’s enough contrast. Checking your design for colour blindness support is also a good idea.
Apple’s iOS and macOS HIGs don’t have much to say on the topic, but they might be worth reading: