Ignore the current rating of the mosfet. Instead look at the Rds(on) value measured at Vgs=4.5V and Id at least as high as the current you want to switch. For the irl8743 used in the SevenSwitch it is 3.9 milliohms at 4.5V and 20A. Multiply that by the square of the current to get the power dissipation. To avoid needing a heatsink, you want the power dissipation to be no more than 1W. That's why the SevenSwitch is rated at 15A. To carry 20A you want the Rds(on) to be no more than 2.5 milliohms.
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