The symbol you wish to use, 5 are predefined, the rest can be found in the GKS manual.
Additional Information on:
The size of the marker. Note that this is a very badly defined quantity, the actual size depends largely on the output device which is used.
The colour used for plotting the markers. Two colours are known on every workstation: BACKGROUND and FOREGROUND. If your workstation allows, you may add further colours yourself using !COLOUR.