You are probably trying to re-open the filestream, try this.
Dim fileStream As New IO.FileStream(mapPath & "\" & filename, FileMode.OpenOrCreate)
If (fileStream.Length) > 50000 Then
System.Web.HttpContext.Current.Response.Write("{success:false, errmsg:'File size is large imagesize}")
fileStream.Close()
Exit Sub
End If
Good luck.