RadSplitter calculates the size of its components and relies on a pixel values to be fully functional. For example, features that require this are MinWidth, MaxWidth, MinHeight, MaxHeight, Scrolling.
Setting height:auto !important to the pane and splitter with CSS is possible solution, but this will approach can be used for very specific scenarios. If this solution works for you you can use it but we do not recommend this approach.
the Telerik team