Play/Script

Two questions:

1. How visual should a play be? They've got to rely on the cheapest of tools, good dialogue. But I think visually. How can I put that in my script without stepping on the director's toes?

2. How big is too big, for a play? How many scene changes are too many? How many extras/characters? For a film script, my story is relatively small scale/low budget. But for a play...what's the proper scale?