How to estimate the required PCB surface area for SMD power resistors
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."