• Length (touchline): minimum 90 m (100 yds), maximum 120m (130 yds)
• Length (goal line): minimum 45 m (50 yds), maximum 90m (100 yds)”
So, a field can be almost square at 90m × 89m or approaching thrice as long as wide, at 120m × 45m.
Reason for this is prior art that can be hard to change (if there’s a stadium around your field, and it’s deemed too small, you’d have to demolish it to make the field fit the standard)
Throw in Australian Rules Football fields if you're looking for a maximum, particularly if orginal marn-grook is in the mix.