What size is the iPhone 6 / iPhone 6 Plus SIM card?
Apple has not changed the size of the SIM card on the iPhone 6 or iPhone 6 Plus. The SIM slot accepts a Nano-SIM card, same size as the iPhone 5 and iPhone 5s.
Apple moved to have the Nano-SIM standard approved to save space in the hardware design of the iPhone 5. The Nano-SIM is essentially a Micro-SIM card with extraneous plastic trimmed from around the outside of the circuitry.
An adaptor can be used to fit a Nano-SIM card into an older device, such as an iPhone 4.