Por lo que entendi yo haria lo siguiente:
Sub Main()
dim StrAuz As String
StrAux="ABC2540"
Msgbox LargoParteNumerica
End Sub
Private Function LargoParteNumerica(Byval s As String) As Integer
dim a As String
dim I As Integer
a=""
For I=1 To Len(s)
If Isnumeric(Mid(s,I,1)) thena=a & Mid(s,I,1)
End If
Next I
LargoParteNumerica=Len(Trim(a))
End Function