Cap Value for Full Wave Rectifier Circuit
I assume you're talking about C5. This capacitor makes sure that the voltage input of the 7812 is a smooth one. It works like this:



Image Reference: http://www.electronics-tutorials.ws/diode/diode_6.html

The heavier the load (higher current), the faster the capacitor discharges, thus the more ripple there will be. We want an input voltage (to the load) as smooth as possible because sometimes a device acts weird due to a power supply with too large of a ripple. 

When the load is so heavy that the ripple is too large, you can use a bigger capacitor, because that smooths the line more. Therefore, it depends on the load what value you need for a capacitor. 

You can calculate the capacitance needed for the capacitor for a given mains frequency (not so important) and load current - however, I'd prefer testing around a bit and measure the ripples with a scope.