Hi Maldwyn,
PDF files are the size of a page (e.g. A4 or Letter) which is why a drawing imported from PDF will usually appear to be small; the original dimensions of the drawing are lost (which is just one reason why exchanging drawings using PDF is usually a terrible idea, but architects and others seem to be doing it more and more anyway).
You can use
Arrange | Scale to resize the drawing and there's a movie that should cover all you need with regard to this topic; see "
Scale Imported PDFs", right at the bottom of
https://www1.stardraw.com/stardraw/sd7/moviesI hope this is helpful.