Type "/33change" and a menu will drop down allowing you to choose 10 colors, 3 sizes or turn the particles off.
You can also use voice commands on channel 33, as follows:
/33 on_off /33 small /33 medium /33 big /33 white /33 black /33 gray /33 blue /33 red /33 green /33 yellow /33 cyan /33 magenta /33 rainbow