Type "/66change" and a menu will drop down allowing you to choose 10 colors, 3 sizes or turn the particles off.
Caution: If you choose "finalize" in dialog box then the color and size is fixed. (even if you type "/66change", dialog box won't drop down)
You can also use voice commands on channel 66, as follows:
/66 on_off /66 finalize /66 small /66 medium /66 big /66 white /66 black /66 gray /66 blue /66 red /66 green /66 yellow /66 cyan /66 magenta /66 rainbow