The size of a football field would be measured in square metres (1m*1m) = 1m2.But, if you're trying to measure one-dimension value then you will use metres.Standard "Gridiron" football field is about 5390m2.
The most appropriate metric unit would be the centimeter.
metres or kilometres
Metres, to measure its length or width, square metres to measure its area.
Officials use yards, as a football field is 100 yards.
Neither. I would use a centimetre.