The surface area is included in the rating of your panel. An 1Wp-panel gives 1W at 1kW/m2 sun intensity, this can be reached by a bigger panel with low efficiency or a smaller one with higher \eta. So for 1kWh/m2 of irradiation energy your panel gives 1Wh, which means for 50kWh/m2 a month it gives 50Wh.

Is this more clear?

EDIT: As said before, efficiency is in reality not independent of light density, and also the sun spectrum changes during the day which has different effects on different cell types (depends mainly on the band gap), so this is a simplification assuming constant light conversion efficiency.