I have a PDF pattern file that I opened in Affinity Designer. Checked the scale of the test box and it is correct.  Exported as SVG. Imported the SVG under File>Import Image. 

Upon opening the file, I get the dialog box asking if I want to scale to fit on canvas or display it in the original size. No matter which one I select, the scale is overblown and takes some manipulating to get the size correct. 

I have my settings in Affinity and XCS set to inches. 

Thanks,