I just came across this post and I see its still active almost 10 years later so I thought I would help out. This is a really goofy implementation in an interface but you have to left click the “Black space” that’s next to your emitter modules to see the menu that Eric Ketchum was talking about.
Then the details panel will show a different menu which doesn’t show up any other way. In that menu is the particle bounds settings.
As old as this is, it still this way in Unreal 5.1.1