Is a 5mm LED brighter than a 3mm?

The candela is the measurement of how much light there is given from a source. The mcd, or millicandela, is just a small part of a candela. It's 1/1000 of a candela just like a millimeter is 1/1000 of a meter. Read the mcd rating of the LED and you'll know how bright it is in comparison to another LED. If the mcd is the same, they're the same brightness.