The size of a particle is limited by the hardware. So you can't scale a particle extremely large - the limit of the size parameter is 16, but some 3D cards support not even that. You can use the DXCaps viewer to check the maximum point size of your 3D card.

When a particle has no bitmap, an internal bitmap with the size 16x16 is generated. That's the default dot image. If you assign an own bitmap, it's scaled up or down dependent on whether it's smaller or larger than 16x16. Hope this helps.