Particles set to facing camera and with 33% blur applied. Trying different sizes with default shader applied. Seems that the particle size to go with depends on how the scene itself is scaled. So this will be something that will need to be adjusted from scene to scene.

Another thing to note, sometimes the particle size changes don't seem to register unless you change some other variables in the particle emitter settings. I find that switching one of the toggle settings back and forth works for this. (Not sure if this is only a 5.1 bug, or something that actually carrys on into the latest version.)

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.