Decision of micro-mapping a gas station is a matter of how detailed you like it to be and is indirectly asking who will benefit from those details in &OpenStreetMap.
If you think of a large station, it could be made of pumps, shop, technical buildings, parkings, picnic sites, decorative grass, bushes, flower beds, etc.
Then, it becomes clearer the filling area is that: the filling area. So, shop, picnic sites, grass, etc. are part of the gas station but not of the filling area.
Where that area exactly starts and stops is your judgment call based on visual clues like curbs, painted lines, differences in ground surface and elevation, etc.