Thank you for the clarification.
Unfortunately, you cannot change the size of the selection background. It is related to changing the default Windows caret
height of the TextBox. The caret height is propertional to the font size of the input text and is not available for free modification.
The only customizable property of the caret would be its color - you could set a telerik:RadWatermarkTextBox .Caret
brush once you have generated the template of RadDateTimePicker.
the Telerik team
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