What are the dimensions of an Australian Rules football ground and WHY is its shape uncommon?

There is no specified size, even at the professional level - let alone suburban junior competitions. It is generally an oval, and needs to be long enough on its major axis to accommodate the 50-metre (radius) arcs centered on each goal, as well as a centre square which is supposed to by 50m on a side. This would make the minimum length 150 metres, and the minimum width at the widest point 50 metres. Most grounds are more circular than that - especially those that were designed to handle both AFL and cricket.

For senior football, the playing field is an oval, typically between 135-185 metres (148-202 yd) long goal-to-goal and 110-155 metres (120-170 yd) wide wing-to-wing. Grounds can vary from long and narrow to almost circular, and are not necessarily symmetrical, depending upon how and where the field was constructed. At least 5 metres (5.5 yd) of space between the boundary line and any fence is required for safety.

https://en.wikipedia.org/wiki/Australian_rules_football_playing_field