Thank you for writing.
By default the AutoSize property is true and all layouts automatically calculate their size depending on the desired size of the inner elements. In this mode you cannot set the Size of the elements (in this case RadPageViewStripItems) but you can adjust their size with the MinSize and MaxSize properties and their offset with padding and margin.
If you set AutoSize property to false, the layouts will not calculate their size automatically and you will be able to set the size manually.
I hope this helps. Let me know if you have any additional questions.
the Telerik team