I'm having a hard time understanding how the "size" parameter for a particle type relates to the actual image size. Let's assume my emitter's size is 100% and I have a 128x128 base particle image that I'm using. What value would I have to set the particle type's size parameter to in order to make it exactly the same size as my source image? I tried setting it to 100 but it looks rather small. I played around with it and sort of got a value that looked like it was the same size, but the number I used didn't seem to have any obvious relation to the default image size. Thanks for the help.
Glad you made me take a look at it, because the answer is simple: if all scaling factors are 100%, the particle type "size" value is the size in pixels of the particle texture when drawn.
For a regular emitter, there are 3 properties that determine the size of the particle: emitter zoom, emitter size, and particle type size. (Well, plus particle shape import size.)
As an example, if you create a simple project file that uses a "single" particle, a shape image that is 128 x 128 pixels, and the zoom and emitter size are both set to 100%, setting the particle type "size" to 128 makes the particle 128 x 128 (plus or minus a pixel or two -- hard to tell with the antialiasing going on). Setting particle type "size" to 256 makes the particle 256 x 256. (Assuming that "size variation" is 0, and "size over life" is constant at 100%)
Super emitters add another layer of scaling, as the free emitter type has f-zoom, f-zoom variation, size variation, size, size over life, f-zoom over life, zoom (this is in addition to the super emitter zoom, size, etc. and particle type size, etc.) Basically any property that has "zoom" or "size" in it applies.
Also keep in mind that even though the shape image is 128 x 128, the non-transparent parts of the texture can be smaller. In the example project you create (if you do) I suggest using the 128x128 white square that's in the "default" library.
I hope that answers it. (And I hope that using different shape sizes doesn't invalidate this -- I only tested the one size!)