Why wouldn't you use "В Одесі..." instead of "У Одесі..."? The latter sounds awkward because of the two vowels.
Yep it's not correct
Are there a lot of big people in Odesa? This exercise sounds strange. It is easy to see what is meant, but it's asking to be made into a joke.
Населення Одеси велике should also be acceptable. The English sentence is actually not standard English. It reads like a translation. We would say "Odesa has a big population."
А у Одесі не може бути багато населення? Чи воно лише "велике" та могутнє?
My translation was not accepted, but would it be correct? -- U Odesa ye velyke naselennya.