You can check this example in our online demos: http://demos.telerik.com/silverlight/#TimePicker/FirstLook
You can change how many columns does the dropdown have and thus modify its size like shown in the example . In your case, according to the screenshot you send me, it will be most suitable to reduce the columns to 4 and I think it will look nice.The xaml will look like :
If this is not to your liking you can modify the template or simply reduce the font size using the FontSize
If you need more help please let me know.
the Telerik team