Wondering how much capacitor is enough and how much is too much. Is there a formula for this kind of thing? Physical size is a limiter, so that part of the question is easy. I've got a bunch of 16v caps ranging from 370uf to 5000uf and multiple caps in series packs. Wondering what's the "right" capacity.

Let's say the setup is 17.5 blinky touring car, with an average servo that doesn't tend to cause brownouts.

That can best be answered by the ESC designer. The power capacitor absorbs spikes that can damage the MOSFETs, and the allowable spike voltage is a function of the MOSFET voltage rating. It's usually best to just stick with the stock capacitor(s).

If using non-stock capacitors, make sure that their ripple current rating is sufficient. Again, this is something that the ESC designer has already done for you.

Thanks Howard. Always good to get advice from you.