It seems that you are showing a single instance of the RadWindow control and if this is the case the behavior you are observing is an expected behavior of the RadWindow control. To alter it, you need to set the Width and Height of the RadWindow control to double.NaN every time you open the window.
I'm not sure I understood the second problem correctly, but if you need your window to have initial size you just need to set the Width and Height properties to the desired values.
All the best,
the Telerik team