My answer would be as big as possible. Like Wilds big (10,000 acres) but for purposes of discussion I'm referring more to the walking around portion of the zoo. Personally I like a zoo to take a whole day to see, whereas the majority probably are more like a 4-5 hour excursion. San Diego's 100 acres is pert near the daily limit of most mere mortals, but partially because the layout grew with out the foresight of a 100 year plan on opening day. I would guess that 140 acres would be near the daily limit IF it were laid out to avoid backtracking to see it all. (larger of course if portions of the zoo were seen via trams, buses etc). All zoos are too small when factoring in off display animals, bachelor and multiple groups and rotational display groups/animals.
No zoo should take more than a day to see. For non-enthusiasts it should take less. Actual land size isn’t what’s important, but the length of walking paths and density of exhibits. A physically very large zoo like Nuremberg can nevertheless be traversed reasonably quickly, thanks to a good layout and am emphasis on quality exhibits rather than packing in as many species as possible.
