Use 330 ohms to limit the current to 10mA. Then the relay's coil can be as low as 50 ohms.As what u say,so what value of resistor should i put at my PORTB ??
With the caveat that these MOSFETs are just barely ON with a 5.0V gate voltage. They should usually be OK with a light load and the full 5V Vcc, but it Vcc is less than 5V (e.g. 4.5V) these MOSFETs aren't guaranteed to turn on.To minimize the load current of the MCU you could use a low power MosFet BS170 (N-channel) or BS 250 (P-channel).
They require extremeley low gate current (compared to a bipolar transistor) to conduct fully.