How big are the pendants?
Round pendants are about 13/4 inches in diameter. Rectangular pendants are generally 2-3 inches long. For reference, a quarter is just slightly under an inch in diameter.
Most cords are braided satin and will be adjustable to around 24″. Sometimes waxed or cotton cord is used. Descriptions will indicate cord material and length.
Necklace closures are either adjustable slip-knots, or lobster claw-style closures with 2″ chain for adjustable length.