A resistor is a very poor choice. The way to do this is with some diodes. Each diode pair drops the voltage about .7 volts. To drop it 5 volts, you'd need seven or eight diode pairs.
A more compact way to do this is with bridge rectifiers, you get two diode pairs for each bridge. Here's a diagram of two bridge rectifiers that are the same as four pairs of diodes. I'd use 3 amp bridge rectifiers.
