If I understand you correctly, you are trying to change the size of the thumbnails in the Page Thumbnails section of the RadPdfViewer. We have a good example in our GitHub repository for how to customize the look of RadPdfViewer by adding thumbnail functionality and especially for your case how to set a custom size of the thumbnails using the ThumbnailsConverter class.
<converters:ThumbnailsConverter x:Key="ThumbnailsConverter" ThumbnailsHeight="100" />
In case you are using the PDFViewer`s API for exporting a PDF document page to an image, you can set the thumbnail size when you are Exporting the FIxed Page to Image:
ThumbnailFactory thumbnailFactory = new ThumbnailFactory();
ImageSource imageSource = thumbnailFactory.CreateThumbnail(radFixedPageInstance, new Size(150, 250));
If none of these cases is yours, it would be helpful if you can share more information about the requirements and the expected results.
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