Whilst coding a Mandelbrot explorer, I needed to find the IFrame size in pixels to produce a Bitmap the same size.  After a lot of wasted time searching the net, via trial and error I came up with the following method:

Assuming you're using a Grid to hold your elements:

<Grid Name="myGrid">
   ...
</Grid>

To get the IFrame's size, we get the XAML page's parent, which is a ContentPresenter that holds the width/heigh in pixels:

Dim browser As System.Windows.Controls.ContentPresenter = VisualTreeHelper.GetParent(myGrid.Parent)
Dim width As Integer = browser.ActualWidth
Dim height As Integer = browser.ActualHeight