Try the steps outlined in this FAQ item:
(you'll need to take care of the mm/m correspondency yourself)
Am 05.12.2017 um 17:56 schrieb Philip Rhoades:
If I produce a new drawing on an A4 page with Display and User units in
mm with a scaling factor of 1.0, I can draw a rectangle of XxY and when
I use the Visualise->Measure Path Extension with a Scale Factor of 100
and Length Unit in m - I get exactly the right dimensions for the
rectangle for its perimeter length and area - all good!
However, I have imported a surveyor's map from a PDF file into SVG and I
need to be able to draw rectangles on it as above in mm (like my test)
that will correspond to m in the real world but the drawing is on an A1
page and the Display and User Units are in px and I have tried
everything I can think of and Googled a lot but I can't do what I was
able to do with my little test case.
How can I convert the drawing to be on an A4 page with mm units so the
mm correspond to m and the Measure Path Extension will give me the
correct result in m and sqm?
The original PDF says the scale was 1:300.