You can use the MinHeight on the ScheduleView's GroupHeader MinHeight for the style that the GroupHeaderStyleSelector will select.
I have attached a project that shows how you can set the basic shcedule view settings for the month view to show min. 3 appointments always however to make the other views work you will have to add them from the RadScheduleView's styles.
You can find the ScheduleView resources in your installation of Telerik controls. For example on my pc they are located here:
C:\Program Files (x86)\Telerik\RadControls for Silverlight Q1 2011 SP1\Themes\OfficeBlack\Themes\Office\Black\ScheduleView.xaml
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