Arguably, if you have DC, what is the capacitor there for at all.
I suspect what you are refering to is a capacitor across DC supply rails, it would be there to reduce the effects of the inductance and resistance of the wiring which would both cause a voltage drop if a sudden load was applied. In that instance, the capacitor would be fitted to work as a local store of energy, released to 'fill in' the voltage dip and it would replenish it's charge when the voltage returned to it's higher level.
The value depends on the nature of the load and the impedance back to the power source. Ideally want it to have zero reactance, in other words a perfect energy source but in reality aiming for 10% of the line impeadance is a more practical target to aim for.
Brian.