open 'Configure', then 'Chart', if you have the legend displaying either Bottom or Side, there should be a Text Size Option...
I don't see these options in ArcGIS Online Dashboard. There is no 'Chart'.
Can you send a pic of the item you are trying to change? I assumed you were looking at Chart on the Dashboard...
Did you ever figure out how to do this? I have a dashboard and I'd like to make the legend text smaller...