Thank you for writing.
I'm not sure I understand exactly what you are trying to achieve. RadSlideView stretches to take the available size and it doesn't follow the size of the currently visible item. You can set its Background property to a visible color, if you'd like to see what size it takes. It doesn't resize when the item is changed.
If you need additional assistance, please share some more details on your scenario, so that we can help you further.
All the best,
the Telerik team
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