You must understand your voltage drops. Transformer has unloaded and rated load voltages. You have voltage drops associated with rectifiers, about 1.3v to 2v for a bridge rectifier. Don't forget you get 1.41 time the rms. voltage rating on transformer for peak voltage. Make sure you cap voltage rating can take the unloaded peak voltage.
Based on your regulator minimum input voltage your slump between cycles must not be lower then this.
A full wave rectifier gives you two peaks per input AC cycle. Half wave rectifier you get only one so the filter cap must supply current across a longer time gap.
For 60 Hz input with full wave rectifier you will have 8.333 millisec between peaks. This must be covered by filter cap.
The formula is C =I * dt/dV, where I is the load current. dV is the acceptable voltage slump between the dt time which is the time between recharging peaks (8.33 millisec in the 60 Hz full wave rectifier example)