How big is (Breath of the Wild's iteration of) Hyrule?

The BOTW map is ~60² km or ~37² miles.

This gif/video makes a size comparison between other LoZ maps (and even Skyrim).

Again, this isn't exact, but using measurements in the game (even using Link's height of ~1.7m/5'7"*) we can approximate.

---

^{* Again, this is an approximation - Source}