https://www.paintcodeapp.com/news/ultimate-guide-to-iphone-resolutions
So, basically, it would appear that the default font sizing of the 6.1" XR would be the same as the iPhone 8.
Width of the phones' screens are as follows:
XS Max 69 mm, with 414 points = 6.0 points per mm
8 Plus 68 mm, with 414 points = 6.1 points per mm
XR 65 mm, with 414 points = 6.4 points per mm
XS 62 mm, with 375 points = 6.0 points per mm
8 59 mm, with 375 points = 6.4 points per mm
The more points per mm, the smaller the font size.
I was hoping the default font sizing of the 6.1" XR would be the same as the Max (which has bigger fonts), but in retrospect, that wouldn't make sense in terms of the math. Instead, the XR's 6.1" screen is just a shrunk down version of the Max's 6.5" so everything will be slightly smaller on the XR. However, one can just use Display Zoom mode.