What size resistor

OK so i want to power a LED light from my arduino via a breadboard and i was wondering if i need external power to power the led. The LED is from a pen light and had three 1.5 volt button cell batteries connected to it, I don't want to wait 17-28 business days to get a pack from china and i had this one lying around. i also want to know what size resistor i need. Thanks

Jameco, mouser, digikey and others will have resistors and led's in your hands by Tuesday. Amazon and e-bay will get you stuff almost as fast if you make sure it is shipping from some place close.

Does Radio shack still carry LED's ?

you could try it with a 1k resistor to see what happens, or something around 400-500 ohms.

or set up a transistor and use an external power supply. this is the safest bet with unknown values.

You won't go far wrong with a 220 ohm resistor, higher will be dimmer, lower will be brighter but might exceed recomended current.