There is a tool in VOSWMM to extract the area of a subcatchment. To do so, you can do the following steps.
1- open VOSWMM and click on the tab “Map”

2- click on “Add Layer”
3- select the layer that you have.
You can see that VOSWMM accepts a lot of formats.

5- select “subcatchment” from the Tool Box and draw your desired subcatchment.

You can see the area of your subcatchment on the right side under properties.
If you have a shapefile and you don’t want to draw a new shape you can right-click on the existing shape and use “Update Geometry Parameter” to update the area value.