Hello François Martel,
I used your code to prepare a simple test project, but the RadSplitter became as high and as wide as the IFRAME - please find the pages of my project attached. I used version 2009.1 311 for my test as you have specified and I tested under IE and FF.
Could you please prepare and send me a simple running project, demonstrating the problem you describe so that I can research was is causing it locally?
As to your second question - you cannot position an element in your content page, relative to your main page (the one containing the IFRAME). However, in case you know the position of the IFRAME (top and left) you can calculate and set the correct position of the Menu relative to your IFRAME.
the Telerik team