I just started from scratch a new theme park and I want your opinions on the perfect size for a very well-developed park. Also, would you think if I had a huge park, does it create lag while creating a park?

the size of the park doesnt matter, its the detail you put in the park. obviously it is easier to make a smaller more detailed park than it is to make a large detailed park. as for the lag, all parks start to lag around 10MB in size, 12MB is pretty much the max before the files just wont load anymore