how well can it handle an AD that is close to 50GB in size?
Adaxes should work just fine with your Active Directory.
Does Adaxes update ACL's on the objects?
Adaxes does not modify native Active directory permissions. Also, it does not extend your AD schema. If you uninstall Softerra Adaxes, you can use Active Directory just as you did before the product installation.
Does it basically store an entire copy of the forest (or domains) in the database installed by the product?
No, Adaxes does not store a copy of your Active Directory forests or domains.