Hi Damien,
iPhone3 and 3GS use Helvetica as system font,
while iPhone4, 4s and 5 use Helvetica Neue.
You can read more
here
As font size, it's somewhat complicated,
since mobileControlGet(tControllerId,"fontSize") doesn't return anything {possibly bug}.
And I know next to nothing about xCode environment
I've run a comparison and can say that the default font size is 12.
But this is not a system info, just an estimation based on visual comparison.
I'm pretty sure, though

A more interesting point is no matter what device or screen resolution the app works on, font size is still 12.
It's something about relative point calculation of retina displays, I guess.
Maybe a more experienced developer or an xCoder can explain further more.
Hope it helps...
Best,
~ Ender Nafi
~... together, we're smarter ...~
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macOS Sierra • LiveCode 7 & xCode 8