"It is a large zoo."
Translation:Это большой зоопарк.
"It is" and ""this is" usually in most cases translates as "это" in Russian, so you can't translates this sentence as "он большой зоопарк" (your choice would be "he is a big zoo", it is not proper Russian).
It doesn't work this way in Russian that you wrote. You can say either "это зоопарк, он большой" (it is a zoo, it is big) or "это большой зоопарк" (it is a big zoo). Because if you want to say in Russian that some object exists you should use the word "это + noun", if you want to say that this object has some features you should use the construction "это + adjective + noun":
"Это камера, она профессиональная" = "It is a camera, it is a professional one"
"Это профессиональная камера" = "It is a professional camera"
"Это дерево, оно старое" = "It is a tree, it is old"
"Это старое дерево" = "It is an old tree"