If you use the initial location module you can define a rectangle/box volume to spawn the sprites within. I highly recommend using GPU particles for this case.
Initial Location max 150, 150, 150 min -150, -150, -150 should yield you a 300 unit square volume.
Using local space will only move them around/rotate their spawn positions and transform positions with the local emitter location/rotation axis of the placed actor.
If you want the particles to be larger, you need to change the initial size.
Check out this page for a full description of what all of the modules in Cascade do.
https://docs.unrealengine.com/latest/INT/Engine/Rendering/ParticleSystems/Cascade/index.html#modules
Hope that helps you