I'm not sure about exact footprint size, but there is a Aquatic Center (now owned by the Central Florida YMCA) parallel to I-4 heading east right before the Sand Lake Road exit.
When I was growing up, it was used for Olympic qualifying in swimming and diving events - so that could give you a rough idea if the complex is going to be something more of that style versus another multi-sport building.
It could be a domed baseball stadium and could potentially have a tennant, with the news that the Tampa Bay Rays are possibly looking for a new home - either in the Tampa/St. Pete area or elsewhere. Doubt it's basketball with the Amway being one of the newest arenas in the league, along with there not really being a team in talks to move anywhere. Definitely not big enough for football - they'd need the entire WWoS complex and then some for infrastructure for that.
With soccer on the rise, it could be a domed stadium for soccer ...