Hi,
I would recommend a good heavy ceramic dog waterbowl.
Easy to clean and very hard for the snake to tip.
I don't give mine bowls big enough that than can fit in them or anything like that. And I would worry about the vibrations form the pump disturbing the snake.
As to cage size smaller is often better from the animals point of view.
Have a read through our caresheet and see if it raises any questions.
dr del