I have alot to answer for it seems...
The stock cap (red one?) is rated for 4400uf 25v I believe, so you'd want another one the same (or the same as the stock item if the spec has changed).
I've explained my reasons for doing the mod several times, and the Novak engineers would tend to say you wont see any benefit; it depends alot on your specific setup as to whether its worth doing or not.
If you have a 13lb truck and not-the-best-nimhs, then even gearing down wont really help the hvmaxx or batts, so the extra cap did make a difference for me. If you have a stock weight emaxx and the recommended gearing with good nimhs or lipos, then you wont see any benefit I reckon.
Only thing I will say/ask is that the new red capacitor unit has proven somewhat problematic for many users, so a switch to a normal cap might be better if anything, rather than a pair of caps. A bunch of 4 small caps in parallel to equal the single caps spec would be the best option. That said, if it works well and the temps are fairly normal (110-140 on the esc and motor), then dont go poking around at it & making it angry...