"There is a big population in Odesa."
Translation:У Одесі велике населення.
Why wouldn't you use "В Одесі..." instead of "У Одесі..."? The latter sounds awkward because of the two vowels.
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."