It wouldn't be the first time an actor had a body stunt double to portray the larger, alter form. Example, on Charmed, Julian McMahon played the human version of Cole Turner, and someone else played the demonic form of Balthazar.
Looks like Amber Heard has been cast as Mera for Justice League Part 1. The rumors for this movie are running wild. Apparently, Green Arrow and Black Canary might cameo along with Hal Jordan/Green Lantern. Another suggesting that Patrick Stewart is rumored to play Brainiac. Ra's al Ghul and Talia al Ghul were rumored to appear, might be why they killed Ra's at the end of Arrow season 3.
Mini update: Doing some more digging on some of the casting, reports regarding Willem Dafoe's character has pointed to several possibilities. One reports he is possibly Dr. T.O. Morrow, as scientist at STAR Labs. Another listed others possibles, an aging Green Arrow/Oliver Queen, a descendant of Steve Trevor, Tom Curry (Arthur Curry/Aquaman's father), Henry Allen or Jay Garrick (highly doubt Garrick since they will have the Flash played by Ezra Miller), Martian Manhunter. Though Tom Curry sounds more logical given that Aquaman is in the movie with his wife Mera, and they had Jeffery Deam Morgan playing Thomas Wayne in BvS.