As Steven said, to design a proper power supply you have to know the maximum current consumption of your circuit and then add some headroom (depending also on how accurate the prediction is).
The first thing to do is to check the datasheets of the main components, depending on the kind of circuit: if you have motors, speakers, power LEDs, hungry microcontrollers...Sum up the power consumed by the hungriest components first, and then check what else remains.
You can also use a benchtop supply and measure the maximum consumption of the circuit, but take care of checking the maximum load and then don't trust it too much if it's lower than expected (looking at the datasheets).
Some margin is not only needed for safety, but also because if the supply works always at 100% capacity it will suffer from heat. Oversize it a bit will result in it being cooler and work better.
As always, you have a tradeoff between having enough headroom versus smaller price/size/weight.