Hello Muhammad,
The callout of the tooltip is actually an image and thus in order to make it bigger, you should prepare your own image and set it to be used instead of teh default one. In order to set your own image, you should use the following style:
Note, that you should use the keyword !important in order to give a higher priority to your CSS and to prevent it to be overridden by the default.
Kind regards,
Svetlina
the Telerik team