Assuming your polygons are stored in a geodatabase and are in a projected coordinate system (PCS), you could set a label expression based on the SHAPE_Area field. That field contains the area of your polygon in whatever unit your PCS uses, for example if you are using UTM with meters as the linear unit, SHAPE_Area will give area in square meters. It's also updated automatically as you edit features. You can use the label expression to convert between units, for example from meters to hectares:
Round(([SHAPE_Area] / 10000),1) & " ha"
If you want polygons less than 1 hectare to be labelled red you could create label classes as shown here. Then you would create another class for SHAPE_Area > 10000