We can split the hexagon into 6 equilateral triangles. If the area of the circle is $100pi$, then the radius is $10$. The radius is equal to the height of one of the equilateral triangles. Using $sin60 = \frac{\sqrt3}{2}$, we get the hypotenuse is $\frac{20\sqrt{3}}{3}$, which is also equal to the side length. Using the hexagon formula (or going back to the equilateral triangle formula $\cdot 6$), we get
$\frac{6}{4} \cdot s^2\sqrt{3} \Rightarrow$ $\frac{6}{4} \cdot (\frac{20\sqrt3}{3})^2 \cdot \sqrt3 \Rightarrow$ $\frac{6}{4} \cdot \frac{20\sqrt3}{3} \cdot \frac{20\sqrt3}{3} \cdot \sqrt3 \Rightarrow$ $\boxed{200\sqrt3}$