Seems like it is not about scale. Initially I set the start and end sizes to 15 and 20 - that should make the particles visible (while the emitter is attached to the rootNode, the particles are very big, crowding the whole screen). However, I have found one of your recent posts here: Can you use the guiNode to create a 2D game? and it looks like I have to move along that line - taking all my game to the rootNode, setting the viewport projection to parallel.
EDIT: just as a side note, I have been looking into all the related questions on making 2D with JME3… I think it worth having a tutorial on how to make a fully capable 2D game with it. Still, after spending so much time with 2D projects I am not even sure that I get all things correctly - for example, this unexpected issue with the emitter…