Set the size after the model. You don't need to use anything smaller than 1.0 for the size. That's small enough as it is.
PHP Code:
entity_set_model(ePlasmaBall, "sprites/plasma.spr")//model
entity_set_size(ePlasmaBall, Float:{-1.0, -1.0, -1.0}, Float:{1.0, 1.0, 1.0})//set size
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