Was there a data sheet for the leds you purchased?
If these are just run of the mill LEDs then you can use 3.3 volt drop for white LEDs. If you ran 2 LEDs in series they would drop 6.6 volts together and no resistor would be required. If just one led, then subtract 3.3 from the 6.3 leaving 3 volts left for resistor to drop. Use ohms law to calculate. For red LEDs use 2.0 to 2.2 volt drop per. Blue LEDs use 3.6 volts.