On the aurora emitter, under emitter particles, you will find a start/end size for x and y. This is in meters. 1.0 is one meter wide. This is where you change the size of the actual particle appearance. Size Y scales automatically if left as zero, or you can stretch the Y dimension by supplying a value.
To change the emitter area size, use Emitter Parameters, emitter size. If you want a single particle to emit from the specified point over and over, set the emitter size to zero here.
