You can try adding more paper formats to the PaperFormat object:
import { PaperFormat } from 'Utils'
PaperFormat.my = { width : 10, height: 10 } // size should be in inches
But then you'd have to pass arguments to the server too, which is not supported yet. See PdfExport#export method and processPageIntoPdfBuffer of the server/src/queue.js, you can find a way to override this (or patch)