I too, as others have said, add glitter AFTER heating the plastic mixed with colorant, and highlight powders, UV, glow, etc......to 360ish.
...and I agree glitter, especially some more than others, are highly affected by heat.
....but in terms of glitter size, the size of glitter does not matter based on the size of your baits you are pouring. It matters more what effect you are after....i.e. what you are trying to create the final product to look like...
...and small glitter takes less volume to go a long way vs larger sizes. In other words, you will use WAY less .008 size glitter in say a cup of plastic than you would .035 or .065 glitter to get the same "penetration" or "consistency".