Depletion region's length in diode
In Britney Spears' Guide To Semiconductor Physics, on the page about pn-junctions, you can find this equation:



For your purposes, ignore the final version and just use the first and third parts of the equality

\$C = \dfrac{A e N_0 w_n}{V}\$

This equation comes from an assumption that the p- and n-sides of the device are equally and uniformly doped (\$N_{a} = N_{d}\$), and wn is half the total depletion width. 

If you don't have equal doping, you'll need to work out the depletion width for the n-side and p-side separately using equations (16) and (17) from Brtiney's page. This is not so bad in the relatively common case that one side is extremely highly (degenerately) doped --- which makes the depletion width on that side negligibly small.