RadTabControl won't generate the tab's content until you select it. After you select another tab, the previous selected content will be destroyed and the new content will be loaded. You can alter this by setting the IsContentPreserved property of RadTabControl to True. But even if you set the property you will need to select the tabs at least once in order to load their content.
The easiest way to achieve your requirement would be to set a equally fixed size to elements in each tab item. If this doesn't work for you it is possible to preload the content of the tabs by selecting them one by one. Note that in order for the content to get properly loaded and use its size you will need to call the tab's UpdateLayout() method.
sender, RoutedEventArgs e)
.tabControl.SelectedIndex = originalSelection;
You can find this approach shown in the attached project. Please give it a try and let me know if it helps.
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