I'd like to add that the size of the capacitor probably varies with the combination of ESC and those elements drawing power from the BEC.

From my testing using a Quantum Comp 2 and the KO Propo 2343 servo, I required greater than a 2200 microfarad capacitor in order to not have my Spektrum unit reset when moving the steering wheel rapidly.

3200 microfarads seemed adequate to keep the receiver from resetting... but even that might not be enough to handle the absolute maximum power requirements on the track.

I'm going to stick with the recommended 4700 microfarad cap. It'll be a pain in the ass to fit it into my car... but that's all that can be done.

Remember to try to find a 16v, 10v, (or even perhaps 6.3v) rated capacitor... typically the smaller the voltage rating, the smaller the cap.