Here a solution to display the current size of the ViewState inside the browsers statusbar:
protected override void OnPreRenderComplete(EventArgs e)
"<script>window.status = 'ViewState: ' + document.forms['__VIEWSTATE'].value.length + ' bytes';</script>");
Put this code in your CodeBehind or in your Web-Base-Page (a page derived from System.Web.UI.Page).
If you are not happy with your ViewState Size, take a look at ViewState Numbers and Solutions to reduce ViewState Size as documented here. Keeping the ViewState on the Server (e.g. via SessionState) instead of the client is one way.