Hi Ravi Prakash,
Thank you for your questions.
The idea of RadMessageBox class is to provide an easy replacement of the standard MessageBox dialog. Just in the case of the standard MessageBox, RadMessageBox does not allow you to set custom size or change the button configuration. If you need such customizations, you can implement a custom dialog form using the following Telerik controls: RadForm, RadLabel and RadButton.
Do not hesitate to contact me again if you have any other questions.
the Telerik team
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