No filter. I just change their water out frequently.
I'm not sure the exact size of the container. It's a plastic storage container. It's VERY long and short, and they seem pretty happy. I've filled it up most of the way so there's only about two inches of open air, and then added floating things for them to climb out on.
Here's a good hint for sexing, and the only one that really seems to work:
The male is the one trying to mate with the other toads.
The female is the one who NEVER tries to mate with the other toads.