Check this site out its pretty cool how its laid out and its really simple. The controls stay in the middle and if you stretch the screen it always fills it up:
http://silverlightdesign.nl/
So how do you do that? Well first you need to set the background of your controls that you want to take up the width to stretch. Notice the vertical gradients stretch very nicely like that ( <Rectangle Height="120" VerticalAlignment="Bottom" Stroke="#00000000" Stretch="Fill"> )
Second, you want to make sure you keep the elements in the middle so that they are not fixed. You don't want your elements to left of the screen if the person adjusts the size of the window. For example, I used a stackpanel and keep my buttons alaways in the center of the screen and on the bottom doing this ( HorizontalAlignment="Center" VerticalAlignment="Bottom")
Don't use fixed width stuff other than the granular/leaf level controls. That will get you in trouble quick. For example, you can have a dialog box with a ton of controls laid out however you want..but lay that dialog box in the center and not position it with fixed points (i.e. Canvas.Left="100).
Please "Mark as Answer" if any of my content helped
Bart Czernicki
http://www.silverlighthack.com | http://www.twitter.com/bartczernicki