The limiting factor for me WRT larger parks seems to be what my computer can handle. There's a point where so much is going on in the game that it becomes laggy and jerky for me. Turning up the priority to high on the task manager helps some, but I have a couple of lovely larger franchise zoos (one has 65 species and 380 animals currently, and around 8k guests and around 120,000 m2 developed) that have become animal storage grounds, because they are too frustrating to scroll around and build in. Seems to happen after around 7k guests or so, and around Adjusting maximum guest numbers and turning video settings down can help, but of course larger zoos need more money to run (in franchise or challenge--in sandbox one can turn money off) and can go into the red if guest numbers are cut too much. And I will admit to not enjoying the game as much with low video settings.
Guess I will have to get a newer computer before I can really "finish" a zoo.
They have smoothed out some of the earlier things that made it harder to manage larger zoos play wise, such as being able to change the difficulty setting (which reduces toy boredom). And careful use of work zones helps (though it doesn't completely eliminate) issues with stupid workers who don't feed the animals or clean their habitats.