You don't mention which form of football you are inquiring about.
However, the goalpost to goalpost dimension would normally be referred to as the length, as it is the greater of the two dimensions. The other measurement, from sideline to sideline would be the width.
In international rugby, the length of the pitch must not exceed 100 metres, and the width must not exceed 69 metres.
In international soccer, the length must be between 100 and 110 metres, and the width must be between 64 metres and 75 metres.
In American football, the gridiron is 100 yards long, by 100 feet wide.
In Gaelic football. the field is approximately 137 metres long and 82 metres wide.
In Australian Rules football, the oval is between 135 and 185 metres long, and between 110 and 155 metres wide.
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