Figured it out!
As you’ll see on the emitter settings that I posted above, there was no ‘Emitter Size‘ control available. The only one that you can see, X, is greyed out. By swapping the emitter type to ‘Box‘, I was able to change the ‘Emitter Size‘ dropdown to ‘XYZ Individual‘, which then made all 3 dimensions available.
Switching the emitter type back to ‘Layer’, I can now access the ‘Emitter Size Z‘ setting and turn it down to ‘0’, which rectifies the problem.
Why this value isn’t available right away, I’m not sure.
