The LED linked to is not typical - it has an internal resistor so it can driven from 12V
Right on page 1:
"12V internal resistor"
That resistor is likely (12V - 2.5V)/.02 = 475 ohm
So with 5v: (5V - 2.5V)/475 = 5.2mA - so you don't need a resistor at all, and it will be dimmer than if powered from 12V.