Okay, so some of the Math-fu one this topic got me inspired, so I used the mass of Roshar that @Leiyan got, and the fact that the value of the gravitational field is 0.7 times that on earth to derive the radius of Roshar (say that five times fast!). Might not be too incredibly useful for the topic at hand, but it might be interesting.
So, per Newton's Laws:
g = Gm/r^2
We can set g to be 0.7*g on earth, or 9.81 m/s^2,m to the mass of Roshar, or 2.1 * 10^24 kg, and G is the gravitational constant, or 6.674 *10^-11 Icanneverremembertheunits.
rearrange for r,
r = sqrt(Gm/0.7g))
and we get a value of 4.49 * 10^6 m, or just under 4 and a half million meters in radius.
That's about two-thirds of the radius of the earth, which seems reasonable.
For all you dumb yanks, that's 2790 miles.
Please feel free to double check my work, I might have gotten something wrong.
Also, that just might have been the nerdiest thing I've ever done.