If I understand you correctly you would like to adjust the size of the RadCollapsiblePanel according to the size of RadGridView that is within it. In order to autosize the grid by the height, you should its AutoSize property and RootElement.StretchHorizontally to true. To adjust the collapsible panel's height you should calculate it by considering the height of the other elements that you have in the panel.
For reference I created a sample project based on the provided pictures and information. Could you please refer to it and see how it works.
I hope this helps. Should you have other questions do not hesitate to write back.
Regards,
Nadya
Progress Telerik
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