To achieve your requirement you will need to extract the template of RadNumericUpAndDown and modify the RepeatButtons in the template. Currently, this is the way to go with this customization. Basically, you can find the RepeatButton element with x:Name set to "increase" and "decrease" and set their Width/Height property. Or replace their style.
If you want the size of the buttons to be bigger by default you can use the Windows8Touch
or the Material
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