The D2 line of $^{87}\mathrm{Rb}$ is the transition between the $5^2\mathrm P_{3/2}$ and the $5^2\mathrm S_{1/2}$ states. Its linewidth $\Gamma$ is always quoted to be $\sim 6\ \mathrm{MHz}$.
But both states have hyperfine splittings.
The transition now is between each (allowed) pair of hyperfine states: how do we calculate the linewidth of each of these transitions?
Data sheet for rubidium-87 here.
