Hello Moises Casusol,
If you want to have your Silverlight application to stretch inside the browser's window, you have to remove any Width and Height properties part of your main UserControl as well as of the html or aspx definition of your Silverlight application xap file. This way you will have your Silverlight application stretched with the browser's window.
I hope this will resolve your problem, if you have any other issues please get back to us immediately.
All the best,
the Telerik team