Hello Syed Danish,
In the attached file, you can find an example of RadHTMLPlaceHolder control occupying all free space.
The initial/default size of the RadHTMLPlaceHolder control depends of the free space and the size of its content. If height and width of the control aren't set explicitly, it takes as much space as it needs.
We are not aware of problem with applying the source for the RadHTMLPlaceHolder. In the sample project you can check that the source is applied with only ONE click of the button.
If your project is totally different than the sample, it will be really helpful if you can send us your project in order to examine issues described.
the Telerik team
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