Calculating MOSFET Heat Dissipation onto PCB
From your data sheet, the on resistance is in the single-digit milliohms, as long as you keep your gate voltage up and keep the FETs turned on "hard".  Your concern would be if Vgs ever drops to below 4 volts or so; then your on resistance would start to increase and you would start heating the switch FETs.  Note that data sheet states that the Gate-Source threshold voltage, which is listed at -2.2 volt max, only guarantees -250 uA, and the excellent "On" resistance value is measured with the gate voltage at -4.5 V.

If you are worried about heat dissipation, TI has done a good job on this app note http://www.ti.com.cn/general/cn/docs/lit/getliterature.tsp?baseLiteratureNumber=snva419&amp;fileType=pdf

The salient point from the above document is: "With only natural convection (i.e. no airflow), and no heat sink, a typical two sided PCB with solid copper fills on both sides, needs at least 15.29 cm squared or ≊ 2.37 in squared of area to dissipate 1 watt of power for a 40°C rise in temperature."