The way you have configured the RadSplitter (Width=100%, Height=100%), means that the RadSplitter should become 100% of its parent's width and height. That is why, I would recommend that you remove the RadSplitter and set background-color for its parent DIV element:
|<div style="width:90%; text-align:center;background-color:red;">text</div>
Now, when you open the page in the browser, you will notice how wide and high your DIV is. Adding a RadSplitter with Width=100%, Height=100%, would cause the splitter to become exactly as wide and as high as this DIV.
In case you have a parent splitter, you will have to set ResizeWithParentPane=false for this child splitter. By default, this property is set to "true" and a nested splitter becomes as wide and as high as its parent RadPane and resizes with it.
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