If you place the RadSplitter into a container element in the page you can set its
Width and Height properties to 100%
and this will resize the splitter to fill the dimensions of the container element.
You should set all the panes dimensions in % as well.
The Splitter in the example above is nested into a layer with height set to 500px
and width set to 800px. The Splitter's Width and Height properties are set to
100% to fill the entire layer size.