You can't tell by the size what voltage to use for a LED - but you can tell by the color! The color of light a LED will emit depending on the materials it's made of, and the materials is made of the 'voltage' of the LED.

This is called the 'forward voltage drop (Vf)' and is the voltage you would get if you measured across the pins while it was working. It is NOT the voltage it should be connected to. What is more important is the CURRENT (If) through the LED, which is around 20mA for most normal small LEDs. To power a LED correctly, you would use a supply voltage higher than the Vf of the LED and limit the current with a resistor. The resistor value is calculated by the formula (Supply voltage - Vf) / LED current. No need to work this out yourself as there is a calculator

Color guide:

• Red 1.8V
• Orange 2.2V
• Yellow 2.2V
• White 3.6V
• Green 2.1V
• Blue 3.6V