For others looking to do this:
1) download source code, alter the classes Thomas mentions to be public instead of internal
2) rebuild, and add the build DLL's (i was only able to get it to build in debug mode, fine for my use though)
3) then in your code
Code:
renderer.RenderDocument();
// whatever else you do...
var stuff = renderer.DocumentRenderer.GetRenderInfoFromPage(resolucionLastPage);
var topBoundary = stuff.Last().LayoutInfo.ContentArea.Y; // or cylce through the objects till you find the one u want
var pageHeight = renderer.DocumentRenderer.FormattedDocument.GetPageInfo(resolucionLastPage).Height;