Basically you could go with the 5V option as it is very close to 6.3V (most pins GI voltage).

The resistor will very likely be a 220 ohm or close to it - it should be marked with coloured bands that read from the closest end, red red brown and then another colour that is the tolerance and can be ignored for this application.

Any resistance from about 150 to 470 could be used for 6.3V. Slightly lower in "ohms value" will allow the LED to draw more current = brighter BUT with possibly reduced life, higher value = less current so not as bright but extended life.

For 5V (in this case 6.3) I always use 220 ohms and have never had a failure yet.