How do I calculate capacitor size in relation to LED quantities?
This will not provide an answer on how to calculate the capacitor size, but I'll use the answer space to draw a small model of a NeoPixel strip + Power supply + cable. This might help you understand the role of the capacitor. I'm sorry if you already knew any of this.

Data lines are not shown, etc.







simulate this circuit &ndash; Schematic created using CircuitLab

Ok, so when the strip draws current, current goes through the cable and with the parasitic R of the cable causes a voltage drop (U = R*i, Ohm's law). The longer and/or thinner the cable, higher the parasitic R and more severe the voltage drop.

During these voltage drops, the capacitor is there as a buffer, as the guide suggests. This also means that if the parasitic R of the cable (and of the power supply) is negligible, the capacitor will make little difference. In the same way, its benefits are limited by the Parasitic R of the Strips itself (measure the voltage of a long strip, it will be far below 5V when on, you might even notice some color change).

So while it may be possible to calculate ripple vs capacitor size, I suggest you determine this empirically, if possible watching the 5V line with a scope as the LEDs light up.