The Particle Size expression outputs the X and Y size of a particle sprite. This can then be used to drive some aspect of a Material.
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In the example above, Particle Size is being multiplied into Particle Color. Notice that we are masking the output so that we only use the Green channel, which corresponds to the Y-Axis, or the length of the particles. This means that as the particles stretch out, they become brighter in color. As they shrink, they will become dimmer.