The third position is for hooking up a batter pack to power the PicoTalk. It is not for a servo.
Any standard LEDs will work. The operating voltage of standard LEDs is usually around 1.2 volts. Anything from 1 to 2 volts will work fine. Which is almost every LED out there. They just can't be LED assemblies that already have integrated resistors, such as our nano spots, mini spotlight LEDs, etc.
Each eyeball measures 28mm