A couple of thoughts on this based on Words of Radiance and the speed of highstorms. First, WoR establishes that:
and
(both quotes from chapter 87, hardcover page 1049–1050).
I can't find the exact quote, but somewhere in the book I remember it saying that a highstorm takes about a day to travel across the continent of Roshar. Assuming that the storm travels at roughly the same speed on its way around the planet, it seems like this gives us a (very) rough idea of how big the continent is in relation to the whole planet.