A hectare is about 1.4 football fields. A standard pitch measures roughly 105 × 68 metres (around 7,140 m²), which is about 0.7 hectares. In other words: a hectare is bigger than a football pitch, not the other way round.
That is why the phrase "an area of X football fields burned down" is misleading: it makes surfaces look bigger than they are. When people say "300 football fields", it is really about 210 hectares. Use the map above to see it at real scale.
The dimensions of a football pitch are not fixed. They are set by Law 1 (The Field of Play) of the IFAB Laws of the Game, the body that — together with FIFA — writes the rules of football. According to Law 1:
Normal matches: length (touchline) minimum 90 m (100 yds) – maximum 120 m (130 yds); width (goal line) minimum 45 m (50 yds) – maximum 90 m (100 yds).
International matches: length minimum 100 m (110 yds) – maximum 110 m (120 yds); width minimum 64 m (70 yds) – maximum 75 m (80 yds).
For the maths on this page we use 105 × 68 m (7,140 m²), the size FIFA recommends for elite-competition stadiums and the most common one at big grounds. It is only a reference: as the rules show, a pitch can be quite a bit larger or smaller.