If you open the Stateflow chart, on the tools strip of "Modeling", turn on the "Symbols Pane" and "Property Inspector", you will able to see and edit the properties of the chart data. Select the output data, set the "size" property. Then this warning will disappear.
Stateflow chart is more like "code". It is better to explicitly set the size of the data. 1 means scalar, [m,n] means matrix. -1 means inherit but sometimes it is hard to determine the size of the output data. That is why it is better to specify it.
