Try to get largest face.
Select face ,start a sketch and convert entities, then go to Evaluate and select Section Properties.
You can get area directly from face.
Here is macro I made without thinking of exceptions, units etc... it works with mm.... I would recommend that you add the unit to the macro as well. Since it is currently just coded to prompt mm^2, no matter what the unit really is. Use this as a start and then add more functions to it... (Perhaps you want to push data to custom property, check to see if it is a flat pattern and not a assembly... etc... Note that macro will fail if you try to run it on assembly or drawing.)
Dim swApp As SldWorks.SldWorks
Dim swModel As SldWorks.ModelDoc2
Dim swPart As SldWorks.PartDoc
Dim swBody As SldWorks.Body2
Dim swFace As SldWorks.Face2
Dim arrBody As Variant
Dim arrFace As Variant
Dim i As Integer
Dim k As Integer
Dim lArea As Double
Set swApp = Application.SldWorks
Set swModel = swApp.ActiveDoc
If swModel Is Nothing Then Exit Sub
Set swPart = swModel
arrBody = swPart.GetBodies2(swBodyType_e.swAllBodies, True)
For i = 0 To UBound(arrBody)
Set swBody = arrBody(i)
arrFace = swBody.GetFaces
For k = 0 To UBound(arrFace)
Set swFace = arrFace(k)
If lArea < (swFace.GetArea * 1000 * 1000) Then
lArea = swFace.GetArea * 1000 * 1000
swApp.SendMsgToUser "Largest surface is: " & lArea & " mm^2"