Relatively positive it will be the Corolla's 1.8L, its already registered and homologated for NA sales. Its a Corolla underneath so they get to circumvent the rules for bringing a new model to market.
Mileage numbers will be identical to the Corolla. I don't see many 20 somethings buying Corolla's but that has everything to do with having no money than it does with environment and efficiency.
The smart ones buy used regardless..