When you have the RadSplitter configured in percentages, all its parent elements including the body, form and html should have their size explicitly set either in percentages or pixels in order to let the splitter calculate the size it should take - you can see a sample demo explaining this below:
This being said, you will get the desired behavior by adding the following style to your page:
html, body, form
and also set all parent elements' heights explicitly e.g DIV, TABLE, TR, TD, etc if the splitter is nested in another element.
On a side note, I also suggest to set VisibleDuringInit=false for the splitter - this will hide the initial resizing of the splitter from the end user and you will get a better behavior.
the Telerik team