We'll need to know a little more about your setup to give you an accurate answer, but In a mech mod, the lowest resistance load you can safely go with on a fully charged battery (4.2v) and stay under your battery's 20 amp limit is .21Ω, which will yield 84 watts.
Voltage / Resistance = Current (amps)..........Voltage² / Resistance = Power (watts)
Remember, your resistance can/will change do to a variety of reasons, so it's imperative that you keep a close eye on your coil's resistance fluctuations, especially if you are setting it up to run at your battery's safety threshold.