Perhaps you can create an ItemContainerStyle based on CoverflowItem instead of ListBoxItem which will be one step closer to the right solution. As for the size of the images you can apply ItemTemplate on the CoverFlow. In that ItemTemplate you can position the text and the image and set fixed Width and Height for the image.
I am not sure I understand what do you mean by "Set style at runtime". When are you going to apply that style? Is it going to be different for the different images? If you are using MVVM and the ItemsSource of the RadCoverflowItem than you can create a Width and Height properties on the view model items and bind the Image's Width and Height to the view model Width and Height pretty much as you would bind the Image's Source.
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