If theta goes from theta_1 to theta_2, then the arc length of can be found by L=int_{theta_1}^{theta_2}sqrt{theta^2+1}d theta.
Since r=theta, which gives {dr}/{d theta}=1, we have L=int_{theta_1}^{theta_2}sqrt{r^2+({dr}/{d theta})^2}d theta
=int_{theta_1}^{theta_2}sqrt{theta^2+1}d theta