Please correct my 'Fontpreview size script'

Hi,

I need to set the preference setting for 'Edit--Preference--Type--Font Preview Size--Small' by script

my code is working, but its not show the tick mark on the font preview size box, pls correct it

var myDocument = app.activeDocument;

app.generalPreferences.includePreview=true;

app.generalPreferences.includePreview.previewSize=PreviewSizeOptions.MEDIUM;