How can I get big bubbles when I electrolyse water with detergent?
This might be tricky. The demos I've seen mostly rely on collecting the hydrogen and oxygen, separately or together, and then bubbling them through a soap solution.
If you've got the soap in the electrolyte, you might have a hard time getting anything more than two piles of foam, one of which is slightly flammable. I doubt the hydrogen foam would even rise in air; the volume percentage of gas is too low.