Capacitors are rated for the maximum voltage they are designed to handle. I’m just a hobbyist, but from my reading, you want to install capacitors with a rating of at least 1.5x the voltage you use in your project. That is why I suggested a 25V capacitor. If you happen to find a 35V or 50V or 100V capacitor, it will be bigger, more expensive, and work the same in a 12V circuit as a 25V capacitor. Note that the capacitors in the picture of your Ramps 1.6+ board are rated at 35V volts, though you are running the board at 12V. I’m guessing the board is rated to handle up to 24V.
I don’t have any engineering experience on how to determine the ideal size for the capacitor. I’m just guessing at 3300uf based on playing around with some projects. The next size up is 4700uf. I don’t think it can hurt to use a bigger capacitor, but bigger is more expensive, and if it is not needed…
Note the capacitor is intended to address the issue of a voltage drop due to the spindle kicking on. If a voltage drop is not the cause of your problem, it will not help. If the issue is a voltage drop, a smaller capacitor may not provide enough power to bridge the gap.