It used to be that all SIM-card-enabled phones used the same size SIMs - but not anymore! The iPhone 4 and many advanced Android models require a MicroSIM card instead of a regular SIM card, and the iPhone 5 takes a NanoSIM (as does the Motorola Droid Ultra, and there are probably more to come).

The best way to tell is either by checking your manual OR taking out the SIM card you have:

Regular SIM cards are 2.5 centimeters long.
MicroSIM cards are 1.5 centimeters long.
NanoSIM cards are 1.23 centimeters long.