How big of a light?

Whats the largest light I would be able to power of the Arduino's digital pin? Would I be able to power a lamp? Is there any way to power something bigger?

Thanks for the help.

(I have an Arduino UNO if that helps)

One could power a city if one so desired and could find a large enough switching element.

"a lamp" is no indication of size so perhaps some detail on what you want to do.

For directly powering, the absolute maximum current for an Arduino Uno pin is 40 mA, but you should never actually draw the absolute maximum. 30 mA would be a safe upper limit.

IanTaylorCB: Is there any way to power something bigger? (I have an Arduino UNO if that helps)

Sure. Give the lamp its own adequate power supply and switch with Mosfets, relays, or power transistors driven by the Arduino.

"Give me a lever long enough and a fulcrum on which to place it, and I shall move the world."