The full plan for the area hasn't released yet but Dubai was estimated at approx 425 hectares and Yumeshima Island (where the Japan Expo will be held) is approximately 390 hectates but Yumeshima will be holding a number of other attractions - such as the MGM hotel also featured on ResortX - so it's likely to be a much smaller footprint.
That being said Milan in 2015 was around 200 hectares and the 2010 Expo in Shanghai was the biggest to date at 523 hectares so they are very varied.