What sized balls are used to throw and slam in beyond 5/3/1 routine ?
Personally, I like Dynamax balls. We use a variety of weights but have difference challenges and “battles” when throwing. This is something we did years ago in high school. But the kind of ball isn’t terribly important as long as it is useful and comfortable for you.
Remember the purpose of a med ball is throw explosively - not to hold during ab work or to lazily lob against a wall or to a partner. Med ball work is FULL BODY. There is no isolation of the CNS or isolation in movement.