I protect the sheep in our society from the wolves.
The standard logic is that you need to try to stay as close to 50 as possible.
No maximum size, no limit on Neutrals other than what it says on some cards. There is a Limitations list or Restricted list that you can peruse at FFG's support page for the game. It limits you to a certain numbers of cards from a specified pool of problematic cards. And I think there's now a banned card pool, as well (sadly).
When building your decks, be brutally efficient and do everything you can to avoid a "kitchen sink" mentality, as difficult as it may be to NOT do that.
Good luck and have fun!!
By the "kitchen sink" mentality do you mean not to stuff your deck with the strongest units and build more of a balanced deck? Can you elaborate a little bit.