Unfortunately, our team doesn’t have expertise in Affinity Designer.
If you create a file in a vector design software like Inkscape, you can expect the scale to be maintained when you upload it to the Glowforge app.
I know some of our customers are using Affinity Designer to design. I don’t see an exact match for your question, but I expect you could find a helpful tutorial in the Tips and Tricks area of our community forum. If not, you could post on the forum and see if someone has suggestions for you.
I’m going to close this thread - if you have another question, go ahead and post a new topic.